includes

Specify associations args to be eager loaded to prevent N + 1 queries. A separate query is performed for each association, unless a join is required by conditions.

For example:

users = User.includes(:address).limit(5)
users.each do |user|
  user.address.city
end

# SELECT "users".* FROM "users" LIMIT 5
# SELECT "addresses".* FROM "addresses" WHERE "addresses"."id" IN (1,2,3,4,5)

Instead of loading the 5 addresses with 5 separate queries, all addresses are loaded with a single query.

Loading the associations in a separate query will often result in a performance improvement over a simple join, as a join can result in many rows that contain redundant data and it performs poorly at scale.

You can also specify multiple associations. Each association will result in an additional query:

User.includes(:address, :friends).to_a
# SELECT "users".* FROM "users"
# SELECT "addresses".* FROM "addresses" WHERE "addresses"."id" IN (1,2,3,4,5)
# SELECT "friends".* FROM "friends" WHERE "friends"."user_id" IN (1,2,3,4,5)

Loading nested associations is possible using a Hash:

User.includes(:address, friends: [:address, :followers])

Conditions

If you want to add string conditions to your included models, you’ll have to explicitly reference them. For example:

User.includes(:posts).where('posts.name = ?', 'example').to_a

Will throw an error, but this will work:

User.includes(:posts).where('posts.name = ?', 'example').references(:posts).to_a
# SELECT "users"."id" AS t0_r0, ... FROM "users"
#   LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
#   WHERE "posts"."name" = ?  [["name", "example"]]

As the LEFT OUTER JOIN already contains the posts, the second query for the posts is no longer performed.

Note that #includes works with association names while #references needs the actual table name.

If you pass the conditions via a Hash, you don’t need to call #references explicitly, as #where references the tables for you. For example, this will work correctly:

User.includes(:posts).where(posts: { name: 'example' })

NOTE: Conditions affect both sides of an association. For example, the above code will return only users that have a post named “example”, and will only include posts named “example”, even when a matching user has other additional posts.

# File activerecord/lib/active_record/relation/query_methods.rb, line 231
    def includes(*args)
      check_if_method_has_arguments!(__callee__, args)
      spawn.includes!(*args)
    end