01_script.rb

require 'bundler'
Bundler.setup(:default)

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

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

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

  create_table :comments, force: true  do |t|
    t.integer :post_id
    t.boolean :approved
  end
end

class Post < ActiveRecord::Base
  has_many :comments
  has_many :approved_comments, -> { where(approved: true) }, class_name: "Comment"
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_association_stuff
    Post.includes(:comments, :approved_comments).where(approved_comments: {post_id: 1}).to_a
  end
end

02_query

SELECT
  "posts"."id" AS t0_r0,
  "posts"."likes" AS t0_r1,
  "comments"."id" AS t1_r0,
  "comments"."post_id" AS t1_r1,
  "comments"."approved" AS t1_r2,
  "approved_comments_posts"."id" AS t2_r0,
  "approved_comments_posts"."post_id" AS t2_r1,
  "approved_comments_posts"."approved" AS t2_r2 FROM "posts"

LEFT OUTER JOIN "comments"
  ON "comments"."post_id" = "posts"."id"
LEFT OUTER JOIN "comments" "approved_comments_posts"
  ON "approved_comments_posts"."post_id" = "posts"."id" AND "approved_comments_posts"."approved" = ?

WHERE "comments"."post_id" = ?  [["approved", true], ["post_id", 1]]