Rails 7 adds disable_joins: true option to has_one :through association

In our previous post, we’ve shown how to use the disable_joins option for has_many: :through association. In this post, we would like to show, how that works for the has_one: :through association as well.

Let’s consider the use case where:

• Users can create multiple posts.
• Anyone looking at the post can add a comment - crowd-sourced comments.

In this case, we can store the crowd-sourced comments in a separate database, as comments can grow quickly and may need a different kind of data management approach.

The database.yml can be like below:

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: root
  password: password

development:
  primary:
    <<: *default
    database: rails_demo_development
  crowd_sourced:
    <<: *default
    migrations_paths: db/migrate_crowd_sourced
    database: rails_demo_crowd_sourced_development

The models would look like below:

# app/models/application_record.rb
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
  connects_to database: { writing: :primary, reading: :primary }
end

# app/models/crowd_sourced_record.rb
class CrowdSourcedRecord < ActiveRecord::Base
  self.abstract_class = true
  connects_to database: { writing: :crowd_sourced, reading: :crowd_sourced }
end

# app/models/user.rb
class User < ApplicationRecord
  has_many :posts
  has_one :latest_post, -> { order('posts.created_at desc').limit(1) }, class_name: 'Post'
end

# app/models/user.rb
class Post < ApplicationRecord
  belongs_to :user
  has_many :comments
  has_one  :latest_comment, -> { order('comments.created_at desc').limit(1) }
end

# app/models/comment.rb
class Comment < CrowdSourcedRecord
end

Before

In Rails 6, to fetch the latest comment for a user, we need to add a custom method, as associations can’t join across databases.

# app/models/user.rb
class User < ApplicationRecord
  has_many :posts
  has_one :latest_post, -> { order('posts.created_at desc').limit(1) }, class_name: 'Post'

  def latest_comment
    Comment.order('comments.created_at desc').where(post_id: posts.pluck(:id)).limit(1)
  end
end

Here, we are fetching the most recent comment created on the posts created by the user. As comments are in different a database, need to pluck the post ids.

> User.first.latest_comment
  User Load (0.2ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]
   (0.3ms)  SELECT "posts"."id" FROM "posts" WHERE "posts"."user_id" = $1  [["user_id", 1]]
  Comment Load (0.6ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN ($1, $2) /* loading for inspect */ ORDER BY comments.created_at desc LIMIT $3  [[nil, 1], [nil, 11], ["LIMIT", 1]]
 => #<ActiveRecord::Relation [#<Comment id: 51, post_id: 11, name: "Lesa Boyer", email: "[email protected]", comment: "Bad song!", created_at: "2021-05-25 10:15:12.527229000 +0000", updated_at: "2021-05-25 10:15:12.527229000 +0000">]>

For all such associations across different databases, we will have to add custom methods as above.

After

Rails 7 has added an option in has_one :through association to make it easier to have multiple databases joining. We can use the has_one :through association with the newly added disable_joins: true option like below.

We do need to add one association in the User model for the latest_post in this case.

class User < ApplicationRecord
  has_many :posts
  has_many :comments, through: :posts, disable_joins: true

  has_one :latest_post, -> { order('posts.created_at desc').limit(1) }, class_name: 'Post'
  has_one :latest_comment, through: :latest_post, disable_joins: true
end

class Post < ApplicationRecord
  belongs_to :user
  has_many :comments
  has_one  :latest_comment, -> { order('comments.created_at desc').limit(1) }, class_name: 'Comment'
end

If we try to load the latest_comment association now, it fires 2 separate queries to load the post ids, and the latest comment with those post ids.

> User.first.latest_comment
  User Load (0.3ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]
  Post Pluck (0.6ms)  SELECT "posts"."id" FROM "posts" WHERE "posts"."user_id" = $1 ORDER BY posts.created_at desc  [["user_id", 1]]
  Comment Load (0.6ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN ($1, $2) ORDER BY comments.created_at desc LIMIT $3  [["post_id", 11], ["post_id", 1], ["LIMIT", 1]]
 => #<Comment id: 51, post_id: 11, name: "Lesa Boyer", email: "[email protected]", comment: "Bad song!", created_at: "2021-05-25 10:15:12.527229000 +0000", updated_at: "2021-05-25 10:15:12.527229000 +0000">

This allows us to eager load the latest comment.

> User.includes(:latest_comment)
  User Load (0.3ms)  SELECT "users".* FROM "users" /* loading for inspect */ LIMIT $1  [["LIMIT", 11]]
  Post Load (0.3ms)  SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10) ORDER BY posts.created_at desc LIMIT $11  [["user_id", 1], ["user_id", 2], ["user_id", 3], ["user_id", 4], ["user_id", 5], ["user_id", 6], ["user_id", 7], ["user_id", 8], ["user_id", 9], ["user_id", 10], ["LIMIT", 1]]
  Comment Load (0.3ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" = $1 ORDER BY comments.created_at desc LIMIT $2  [["post_id", 11], ["LIMIT", 1]]
 => #<ActiveRecord::Relation [#<User id: 1, name: "Milton Dicki", email: "[email protected]", created_at: "2021-04-20 17:13:20.492541000 +0000", updated_at: "2021-04-20 17:13:20.492541000 +0000">, #<User id: 2, name: "Crystle Davis", email: "[email protected]", created_at: "2021-04-20 17:13:20.529307000 +0000", updated_at: "2021-04-20 17:13:20.529307000 +0000">, #<User id: 3, name: "Junita Sauer", email: "[email protected]", created_at: "2021-04-20 17:13:20.554967000 +0000", updated_at: "2021-04-20 17:13:20.554967000 +0000">, ...>]>