Multiple foreign keys for the same model in Rails 6

It’s quite easy to create tables with Ruby On Rails, but some cases require a little personalization to reproduce real world situations and the relationships between them. Here, we have two models: User and Meeting. They have a 1..N relation, so one User will be part of a meeting and a Meeting will have two users (yes, specifically two). The problem doesn’t rely on this relationship but in the generated migration because we cannot have two t.references :user_id and we also want to identify which user is available for the meeting and which user who requested the meeting.

Before we start with the solution, let’s understand the commandrails generate or the shorthand coderails g .

This command is one of the many options that follow therails command. If you open your terminal and run rails --help , you’ll see a list of options:

$ rails --help
The most common rails commands are:
 generate     Generate new code (short-cut alias: "g")
 console      Start the Rails console (short-cut alias: "c")
 server       Start the Rails server (short-cut alias: "s")
 test         Run tests except system tests (short-cut alias: "t")
 test:system  Run system tests
 dbconsole    Start a console for the database specified in config/database.yml (short-cut alias: "db")
new          Create a new Rails application. "rails new my_app" creates a new application called MyApp in "./my_app"

And if we run rails generate --help another list of options will be shown:

$ rails g --help                                
Spring preloader in process 70748
Usage: rails generate GENERATOR [args] [options]
...
Please choose a generator below.
Rails:
  application_record
  assets
  channel
  controller
  generator
  helper
  integration_test
  jbuilder
  job
  mailbox
  mailer
  migration
  model
  resource
  responders_controller
  scaffold
  scaffold_controller
  system_test
  task
...

In this article, we will use the rails db:migrate and rails generate model options to create our models and save them on db/schema.rb , the file that represents our database.

First, we will generate the model User and Meeting

$ rails generate model User name:string email:string

$ rails generate model Meeting starts_at:datetime ends_at:datetime available_user:references requester_user:references

They will generate the migration files, the model files under app/models , and the test files. Meeting’s migration should look like this:

Migration to create the table Meeting on database

But if you try to run rails db:migrate to create the tables in the database an error will be raised on your terminal since there aren’t tables called available_user and requester_user to make reference as Foreign Key. With that in mind, our next step is to remove the foreign_key: true , create the foreign key reference with the correct columns, and then the file should look like this:

Now we can safely run rails db:migrate and see the changes in db/schema.rb .

In order to tell Rails what table will be referenced by the Meetings model, we should make changes on our model files app/models/meetings.rb and app/models/users.rb . They should look like this:

Now the relationship is done and we can create meetings and users in this way:

user1 = User.create!(name: 'John Snow', email: 'night_watcher@thegreatwall.com')
=> #<User id: 1, name: "John Snow", email: "night_watcher@thegreatwall.com", created_at: "2020-06-12 20:34:41", updated_at: "2020-06-12 20:34:41">
user2 = User.create!(name: 'Daenerys Targaryen', email: 'stormborn@dragonstone.com')
=> #<User id: 2, name: "Daenerys Targaryen", email: "stormborn@dragonstone.com", created_at: "2020-06-12 20:36:10", updated_at: "2020-06-12 20:36:10">

starting_time = Time.zone.now
ending_time = Time.zone.now + 2.hour
meeting_about_family = Meeting.create!(starts_at: starting_time, ends_at: ending_time, available_user_id: user2.id, requester_user_id: user1.id)
=> #<Meeting id: 1, starts_at: "2020-06-12 20:40:45", ends_at: "2020-06-12 22:41:04", available_user_id: 2, requester_user_id: 1, created_at: "2020-06-12 20:41:41", updated_at: "2020-06-12 20:41:41">

meeting_about_family.available_user
=> #<User id: 2, name: "Daenerys Targaryen", email: "stormborn@dragonstone.com", created_at: "2020-06-12 20:36:10", updated_at: "2020-06-12 20:36:10">
meeting_about_family.requester_user
=> #<User id: 1, name: "John Snow", email: "night_watcher@thegreatwall.com", created_at: "2020-06-12 20:34:41", updated_at: "2020-06-12 20:34:41">