When I started learning Rails, I was always in a state of confusion about using includes or joins, because both are used in almost same scenario. Includes uses eager loading whereas joins uses lazy loading. Both are used when certain operations are meant to be performed on associated tables. And here comes the difference.
What is the problem with Joins?
Here, we retrieved users in the first line and data is formatted in the each do loop. Here is interesting thing, to retrieve user.company.name, each time a call to database is made. It means, if there are 10k users then 10k separate queries would be fired up.
And this will overkill system when scaled up. Isn’t there is efficient way? Yes, it is Includes.
If we go through Rails documenting, it clearly says — ‘With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries’. When we need data to be used from associated tables, includes must be used.
Here is example,
Here, the data from table companies is fetched right in the first line, as it says includes companies. No additional query is fired from the do loop for ‘user.company.name’. Size of users table does not matter now(unless you are not talking about very large dataset that needs efficient data structure as well). It is scalable to any size.
Isn’t it fantastic? Yes, it is. It saves hell lot of time. And it proves to be very efficient, when application contains huge data.
So, does includes always preferred over joins?
Hopefully not, there are some cases when includes does not exceed joins in terms of efficiency.
Here, we used tables users and companies, but companies is only for some sort of filter. We did not actually used data from companies later in the code. So no need to bring data from companies along with users. This is why join is efficient in this case.