Rails EXISTS() subquery

test_rails_exists.rb

# https://github.com/rails/rails/pull/29619#issuecomment-392583498
# Better approach than http://codesnik.github.io/rails/2015/09/03/activerecord-and-exists-subqueries.html
require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  gem 'rails', '5.2'
  gem 'pg'
  gem 'pry'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

ActiveRecord::Base.establish_connection(adapter: 'postgresql', database: 'test_exists')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.references :post
    t.string :text
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class PostCommentsExistsInnerTest < ActiveSupport::TestCase
  def setup
    @post = Post.create!
    @comment = @post.comments.create!(text: 'Foo')

    @post_b = Post.create!

    @query = Post.where(Comment.where('post_id = posts.id').where(text: 'Foo').arel.exists)
  end

  def test_post_is_included
    assert_includes @query, @post
  end

  def test_post_b_is_not_included
    refute_includes @query, @post_b
  end
end

class PostCommentsExistsInterpolatedTest < ActiveSupport::TestCase
  def setup
    @post = Post.create!
    @comment = @post.comments.create!(text: 'Foo')

    @post_b = Post.create!

    exists = Comment.where('comments.post_id = posts.id').where(text: 'Foo')
    @query = Post.where("EXISTS (#{exists.to_sql})")
  end

  def test_post_is_included
    assert_includes @query, @post
  end

  def test_post_b_is_not_included
    refute_includes @query, @post_b
  end
end