Preload, EagerLoad, Includes, and Joins

Optimizing Database Queries in Ruby on Rails: Exploring Preload, EagerLoad, Includes, and Joins.

Ruby on Rails provides several methods — preload, eager_load, includes, and joins — to optimize database queries for associated models. Understanding when to use each one is key to avoiding performance pitfalls like the “N+1 query” problem.

In this post, we’ll explore these querying methods through examples with User, Post, and Comment models. You’ll learn the tradeoffs of each technique and when to apply them to efficiently retrieve related data. By the end, you’ll know how to squeeze the best performance out of Rails database queries by eagerly loading and joining associations.

Eager Loading

Eager loading is a database technique to improve query performance by loading the necessary data and its related entities, including the associated entities, in a single query. It allows the query to load the required resources as soon as the code is executed, reducing the number of queries and improving the overall efficiency of the database.

Eager Loading methods provided by Rails

1. Preload
2. Eager_load
3. Includes
4. Joins

Understanding the above methods

Suppose we have a User model and a Post model. The User model will have a one-to-many association with the Post model, which means that a user can have many posts, but a post can belong to only one user.

The Post and Comment models have the same association as the User and Comment models.

User model

class User < ActiveRecord::Base
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable
         has_many :posts
         has_many :comments
end

Post model

class Post < ActiveRecord::Base
    has_many :comments, dependent: :destroy
    belongs_to :user
    
    validates :title, presence: true
    validates :content, presence: true, length: { minimum: 10 }
end

Comment model

class Comment < ActiveRecord::Base
    belongs_to :post
    belongs_to :user

    validates :comment, presence: true, length: { minimum: 1 }
    validates :post, presence: true
end

As an illustrative example, let’s consider that we’ve populated our database with 4 users, 9 posts, and 0 comments.

Preload

Pros: Preload loads associated records in a separate query, reducing the number of database queries and improving performance. It is useful when you want to access the associated records but do not need them immediately.

If we want to retrieve all posts with their associated users, we can use the preload method.

Post.preload(:user).limit(5)

Output :

Post Load (0.3ms)  SELECT  "posts".* FROM "posts" LIMIT 5
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN (1, 2)

=> #<ActiveRecord::Relation [#<Post id: 1, title: "Post 1", content: "hi everyone", created_at: "2023-04-04 12:50:32", updated_at: "2023-04-04 12:50:32", user_id: 1, email: nil>, #<Post id: 2, title: "Post 2", content: "hello post 2\r\n", created_at: "2023-04-04 12:50:48", updated_at: "2023-04-04 12:50:48", user_id: 1, email: nil>, #<Post id: 3, title: "Post 3", content: "hello Post3 ", created_at: "2023-04-04 12:51:03", updated_at: "2023-04-04 12:51:03", user_id: 1, email: nil>, #<Post id: 4, title: "post 4", content: "hello post 4", created_at: "2023-04-04 12:51:56", updated_at: "2023-04-04 12:51:56", user_id: 2, email: nil>, #<Post id: 5, title: "Post 5", content: "hello Post 5", created_at: "2023-04-04 12:52:10", updated_at: "2023-04-04 12:52:10", user_id: 2, email: nil>]>

We can also access the user related to the post.

Post.preload(:user).limit(5).first.user

Output:

#<User id: 1, email: "rails@test.com", created_at: "2023-04-04 12:49:30", updated_at: "2023-04-04 12:49:30">

Cons: 1) Preload can lead to N+1 query problems, where you end up making one query for the main records and then one query per associated record. This can result in slower performance than expected.

2) While preload can be useful for loading associations in a separate query, one drawback is that it cannot be used with a where clause. This is because preload executes a separate query to retrieve the associated records, which means that any conditions specified in the where clause would not be applied to the associated records.

Post.preload(:user).where(users: {id: 1})

Output:

ActiveRecord::StatementInvalid (SQLite3::SQLException: no such column: users.id: SELECT "posts".* FROM "posts" WHERE "users"."id" = ?)

Eagerload

In Ruby on Rails, eager_load is a method that can be used to eager load associations when retrieving records from the database, but the catch is that it uses LEFT OUTER JOIN for executing its query.

Pros: 1) EagerLoad loads all associated records in a single query, which can significantly improve performance. It is useful when you know you will need the associated records and want to avoid N+1 query problems.

If we want to retrieve all posts with their associated users, we can use the eager_load method.

Post.eager_load(:user).limit(5)

Output:

SELECT  "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."content" AS t0_r2, "posts"."created_at" AS t0_r3, "posts"."updated_at" AS t0_r4, "posts"."user_id" AS t0_r5, "posts"."email" AS t0_r6, "users"."id" AS t1_r0, "users"."email" AS t1_r1, "users"."encrypted_password" AS t1_r2, "users"."reset_password_token" AS t1_r3, "users"."reset_password_sent_at" AS t1_r4, "users"."remember_created_at" AS t1_r5, "users"."created_at" AS t1_r6, "users"."updated_at" AS t1_r7 FROM "posts" LEFT OUTER JOIN "users" ON "users"."id" = "posts"."user_id" LIMIT 5

=> #<ActiveRecord::Relation [#<Post id: 1, title: "Post 1", content: "hi everyone", created_at: "2023-04-04 12:50:32", updated_at: "2023-04-04 12:50:32", user_id: 1, email: nil>, #<Post id: 2, title: "Post 2", content: "hello post 2\r\n", created_at: "2023-04-04 12:50:48", updated_at: "2023-04-04 12:50:48", user_id: 1, email: nil>, #<Post id: 3, title: "Post 3", content: "hello Post3 ", created_at: "2023-04-04 12:51:03", updated_at: "2023-04-04 12:51:03", user_id: 1, email: nil>, #<Post id: 4, title: "post 4", content: "hello post 4", created_at: "2023-04-04 12:51:56", updated_at: "2023-04-04 12:51:56", user_id: 2, email: nil>, #<Post id: 5, title: "Post 5", content: "hello Post 5", created_at: "2023-04-04 12:52:10", updated_at: "2023-04-04 12:52:10", user_id: 2, email: nil>]>

The LEFT OUTER JOIN used by eager_load can cause duplicate data to be returned, as well as slow down performance when working with large datasets. This is because the LEFT OUTER JOIN joins all records from both tables, regardless of whether they have an association or not.

2) It can be used with a where clause.

Post.eager_load(:user).where(users: {id: 1} ).count

Output:

SELECT COUNT(DISTINCT "posts"."id") FROM "posts" LEFT OUTER JOIN "users" ON "users"."id" = "posts"."user_id" WHERE "users"."id" = ?  [["id", 1]]

=> 3

Cons: 1) EagerLoad can be memory-intensive, especially if there are many associated records. It can also lead to more complex queries and potentially slower performance if not used correctly.

2) The LEFT OUTER JOIN used by eager_load can cause duplicate data to be returned, as well as slow down performance when working with large datasets. This is because the LEFT OUTER JOIN joins all records from both tables, regardless of whether they have an association or not.

Includes

Pros: 1) Includes loads associated records in a separate query, like Preload, but also includes the main records in the same query. This can improve performance and reduce the risk of N+1 query problems.

If we want to retrieve all posts with their associated users, we can use the Includes method to eager load the associated users.

Post.includes(:user)

Output:

Post Load (0.7ms)  SELECT "posts".* FROM "posts"
  User Load (0.1ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN (1, 2, 3)

=> #<ActiveRecord::Relation [#<Post id: 1, title: "Post 1", content: "hi everyone", created_at: "2023-04-04 12:50:32", updated_at: "2023-04-04 12:50:32", user_id: 1, email: nil>, #<Post id: 2, title: "Post 2", content: "hello post 2\r\n", created_at: "2023-04-04 12:50:48", updated_at: "2023-04-04 12:50:48", user_id: 1, email: nil>, #<Post id: 3, title: "Post 3", content: "hello Post3 ", created_at: "2023-04-04 12:51:03", updated_at: "2023-04-04 12:51:03", user_id: 1, email: nil>, #<Post id: 4, title: "post 4", content: "hello post 4", created_at: "2023-04-04 12:51:56", updated_at: "2023-04-04 12:51:56", user_id: 2, email: nil>, #<Post id: 5, title: "Post 5", content: "hello Post 5", created_at: "2023-04-04 12:52:10", updated_at: "2023-04-04 12:52:10", user_id: 2, email: nil>, #<Post id: 6, title: "Post 6", content: "hello post 6", created_at: "2023-04-04 12:52:41", updated_at: "2023-04-04 12:52:41", user_id: 2, email: nil>, #<Post id: 7, title: "post 7", content: "hello post 7", created_at: "2023-04-04 12:53:51", updated_at: "2023-04-04 12:53:51", user_id: 3, email: nil>, #<Post id: 8, title: "post 8", content: "hello post 8", created_at: "2023-04-04 12:54:04", updated_at: "2023-04-04 12:54:04", user_id: 3, email: nil>, #<Post id: 9, title: "post 9", content: "hello post 9", created_at: "2023-04-04 12:54:19", updated_at: "2023-04-04 12:54:19", user_id: 3, email: nil>]> 

2) It can be used with a where clause.

Post.includes(:user).where(users: {id: 1}).count

Output:

SELECT COUNT(DISTINCT "posts"."id") FROM "posts" LEFT OUTER JOIN "users" ON "users"."id" = "posts"."user_id" WHERE "users"."id" = ?  [["id", 1]]

=> 3

This method in Rails is actually a combination of eager loading and preloading.

As we use the WHERE clause, it uses LEFT OUTER JOIN of eager loading. When it comes to normal fetching of associated data, it uses preload.

Joins

Pros: 1) Joins combine tables from multiple models into a single query, which can be more efficient than making multiple queries. It is useful when you need to filter or order based on associated records.

It uses inner join for the above feature.

If we want to retrieve all posts with their associated users, we can use the joins method to eager load the associated users:

Post.joins(:user).limit(5)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

Output:

SELECT  "posts".* FROM "posts" INNER JOIN "users" ON "users"."id" = "posts"."user_id" LIMIT 5

=> #<ActiveRecord::Relation [#<Post id: 1, title: "Post 1", content: "hi everyone", created_at: "2023-04-04 12:50:32", updated_at: "2023-04-04 12:50:32", user_id: 1, email: nil>, #<Post id: 2, title: "Post 2", content: "hello post 2\r\n", created_at: "2023-04-04 12:50:48", updated_at: "2023-04-04 12:50:48", user_id: 1, email: nil>, #<Post id: 3, title: "Post 3", content: "hello Post3 ", created_at: "2023-04-04 12:51:03", updated_at: "2023-04-04 12:51:03", user_id: 1, email: nil>, #<Post id: 4, title: "post 4", content: "hello post 4", created_at: "2023-04-04 12:51:56", updated_at: "2023-04-04 12:51:56", user_id: 2, email: nil>, #<Post id: 5, title: "Post 5", content: "hello Post 5", created_at: "2023-04-04 12:52:10", updated_at: "2023-04-04 12:52:10", user_id: 2, email: nil>]>

2) It can also be used with where clause.

Post.joins(:user).where(users: {id: 1} ).count

Output:

SELECT COUNT(*) FROM "posts" INNER JOIN "users" ON "users"."id" = "posts"."user_id" WHERE "users"."id" = ?  [["id", 1]]

=> 3

Cons) There are some drawbacks of using the Joins model, as it uses INNER JOIN:

1. It may give a duplicate record in a few cases.
2. It may result in empty records if an associated record does not have data, which can be represented as a null or empty value.

We will take the example of post and comment as it has 0 comments.

Post.joins(:comments)

Output:

#<ActiveRecord::Relation []>

And if we use any of the above methods (Preload, eager_load, Includes), it will show the post as follows:

Post.includes(:comments)

Output:

SELECT "posts".* FROM "posts"
  Comment Load (0.3ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN (1, 2, 3, 4, 5, 6, 7, 8, 9)
=> #<ActiveRecord::Relation [#<Post id: 1, title: "Post 1", content: "hi everyone", created_at: "2023-04-04 12:50:32", updated_at: "2023-04-04 12:50:32", user_id: 1, email: nil>, #<Post id: 2, title: "Post 2", content: "hello post 2\r\n", created_at: "2023-04-04 12:50:48", updated_at: "2023-04-04 12:50:48", user_id: 1, email: nil>, #<Post id: 3, title: "Post 3", content: "hello Post3 ", created_at: "2023-04-04 12:51:03", updated_at: "2023-04-04 12:51:03", user_id: 1, email: nil>, #<Post id: 4, title: "post 4", content: "hello post 4", created_at: "2023-04-04 12:51:56", updated_at: "2023-04-04 12:51:56", user_id: 2, email: nil>, #<Post id: 5, title: "Post 5", content: "hello Post 5", created_at: "2023-04-04 12:52:10", updated_at: "2023-04-04 12:52:10", user_id: 2, email: nil>, #<Post id: 6, title: "Post 6", content: "hello post 6", created_at: "2023-04-04 12:52:41", updated_at: "2023-04-04 12:52:41", user_id: 2, email: nil>, #<Post id: 7, title: "post 7", content: "hello post 7", created_at: "2023-04-04 12:53:51", updated_at: "2023-04-04 12:53:51", user_id: 3, email: nil>, #<Post id: 8, title: "post 8", content: "hello post 8", created_at: "2023-04-04 12:54:04", updated_at: "2023-04-04 12:54:04", user_id: 3, email: nil>, #<Post id: 9, title: "post 9", content: "hello post 9", created_at: "2023-04-04 12:54:19", updated_at: "2023-04-04 12:54:19", user_id: 3, email: nil>]>
irb(main):021:0>

3. There is also a drawback which is related to the (n + 1) issue that it does not solve because it does not store associated data for future use.

For example,

post = Post.joins(:user).limit(5)

# when we want to find the first post user
post.first.user

Output:

# output of post = Post.joins(:user).limit(5)
Post Load (0.1ms)  SELECT  "posts".* FROM "posts" INNER JOIN "users" ON "users"."id" = "posts"."user_id" LIMIT 5
=> #<ActiveRecord::Relation [#<Post id: 1, title: "Post 1", content: "hi everyone", created_at: "2023-04-04 12:50:32", updated_at: "2023-04-04 12:50:32", user_id: 1, email: nil>, #<Post id: 2, title: "Post 2", content: "hello post 2\r\n", created_at: "2023-04-04 12:50:48", updated_at: "2023-04-04 12:50:48", user_id: 1, email: nil>, #<Post id: 3, title: "Post 3", content: "hello Post3 ", created_at: "2023-04-04 12:51:03", updated_at: "2023-04-04 12:51:03", user_id: 1, email: nil>, #<Post id: 4, title: "post 4", content: "hello post 4", created_at: "2023-04-04 12:51:56", updated_at: "2023-04-04 12:51:56", user_id: 2, email: nil>, #<Post id: 5, title: "Post 5", content: "hello Post 5", created_at: "2023-04-04 12:52:10", updated_at: "2023-04-04 12:52:10", user_id: 2, email: nil>]>


# output of post.first.user
SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1  [["id", 1]]
=> #<User id: 1, email: "rails@test.com", created_at: "2023-04-04 12:49:30", updated_at: "2023-04-04 12:49:30">

As shown in the output, using joins results in an additional query to retrieve the user associated with the first post. This is because joins does not store the preloaded associated data for future use.

For example,

post = Post.includes(:user).limit(5)

# when we want to find the first post user
post.first.user

Output:

# output of post = Post.includes(:user).limit(5)
SELECT  "posts".* FROM "posts" LIMIT 5
  User Load (0.1ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN (1, 2)
=> #<ActiveRecord::Relation [#<Post id: 1, title: "Post 1", content: "hi everyone", created_at: "2023-04-04 12:50:32", updated_at: "2023-04-04 12:50:32", user_id: 1, email: nil>, #<Post id: 2, title: "Post 2", content: "hello post 2\r\n", created_at: "2023-04-04 12:50:48", updated_at: "2023-04-04 12:50:48", user_id: 1, email: nil>, #<Post id: 3, title: "Post 3", content: "hello Post3 ", created_at: "2023-04-04 12:51:03", updated_at: "2023-04-04 12:51:03", user_id: 1, email: nil>, #<Post id: 4, title: "post 4", content: "hello post 4", created_at: "2023-04-04 12:51:56", updated_at: "2023-04-04 12:51:56", user_id: 2, email: nil>, #<Post id: 5, title: "Post 5", content: "hello Post 5", created_at: "2023-04-04 12:52:10", updated_at: "2023-04-04 12:52:10", user_id: 2, email: nil>]>

# output of post.first.user
#<User id: 1, email: "rails@test.com", created_at: "2023-04-04 12:49:30", updated_at: "2023-04-04 12:49:30">

Conclusion

In general, the fastest method for retrieving associated records in Ruby on Rails is Joins, followed by eager_load, Preload, and finally, Includes. However, the choice of method depends on the specific requirements of your application and the relationships between your models.

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