[Rails] Eager Loading Associations

Eager loadings the mechanism for loading the associated records of the objects returned by Model.find using as few queries as possible.

N + 1 queries problem

Consider the following code, which finds 10 books and prints their authors’ last_name:

books = Book.limit(10)

books.each do |book|
  puts book.author.last_name
end

This code looks fine at the first sight. But the problem lies within the total number of queries executed. The above code executes 1 (to find 10 books) + 10 (one per each book to load the author) = 11 queries in total.

Solution to N + 1 queries problem

Active Record lets you specify in advance all the associations that are going to be loaded.

includes

With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.

Revisiting the above case using the includes method, we could rewrite Book.limit(10) to eager load authors:

books = Book.includes(:author).limit(10)

books.each do |book|
  puts book.author.last_name
end

The above code will execute just 2 queries, as opposed to the 11 queries from the original case:

SELECT `books`.* FROM `books` LIMIT 10
SELECT `authors`.* FROM `authors`
  WHERE `authors`.`book_id` IN (1,2,3,4,5,6,7,8,9,10)

Eager Loading Multiple Associations

Active Record lets you eager load any number of associations with a single Model.find call by using an array, hash, or a nested hash of array/hash with the includes method.

Array of Multiple Associations

Customer.includes(:orders, :reviews)

This loads all the customers and the associated orders and reviews for each.

Nested Associations Hash

Customer.includes(orders: {books: [:supplier, :author]}).find(1)

This will find the customer with id 1 and eager load all of the associated orders for it, the books for all of the orders, and the author and supplier for each of the books.

Specifying Conditions on Eager Loaded Associations

Even though Active Record lets you specify conditions on the eager loaded associations just like joins, the recommended way is to use joins instead.

However if you must do this, you may use where as you would normally.

Author.includes(:books).where(books: { out_of_print: true })

This would generate a query which contains a LEFT OUTER JOIN whereas the joins method would generate one using the INNER JOIN function instead.

  SELECT authors.id AS t0_r0, ... books.updated_at AS t1_r5 FROM authors LEFT OUTER JOIN "books" ON "books"."author_id" = "authors"."id" WHERE (books.out_of_print = 1)

preload

books = Book.preload(:author).limit(10)

books.each do |book|
  puts book.author.last_name
end

The above code will execute just 2 queries, as opposed to the 11 queries from the original case:

SELECT `books`.* FROM `books` LIMIT 10
SELECT `authors`.* FROM `authors`
  WHERE `authors`.`book_id` IN (1,2,3,4,5,6,7,8,9,10)

The preload method uses an array, hash, or a nested hash of array/hash in the same way as the includes method to load any number of associations with a single Model.find call. However, unlike the includes method, it is not possible to specify conditions for preloaded associations.

eager_load

With eager_load, Active Record loads all specified associations using a LEFT OUTER JOIN.

books = Book.eager_load(:author).limit(10)

books.each do |book|
  puts book.author.last_name
end

The above code will execute just 2 queries, as opposed to the 11 queries from the original case:

SELECT DISTINCT `books`.`id` FROM `books` LEFT OUTER JOIN `authors` ON `authors`.`book_id` = `books`.`id` LIMIT 10
SELECT `books`.`id` AS t0_r0, `books`.`last_name` AS t0_r1, ...
  FROM `books` LEFT OUTER JOIN `authors` ON `authors`.`book_id` = `books`.`id`
  WHERE `books`.`id` IN (1,2,3,4,5,6,7,8,9,10)

The eager_load method uses an array, hash, or a nested hash of array/hash in the same way as the includes method to load any number of associations with a single Model.find call. Also, like the includes method, you can specify conditions for eager loaded associations.

Reference

• https://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations