Condensed reproduction of issue described in https://github.com/rails/rails/issues/9303. Also see original repro: https://github.com/tlatim/arel_sum/blob/master/spec/models/book_spec.rb

book_spec.rb

require 'spec_helper'

describe Book do
  it "has same join behavior for logically equivalent subqueries" do
    author = Author.create
    book = Book.create!(cost: 100, author: author)
    2.times { Chapter.create!(book: book) }

    sub_query1 = Book.arel_table[:author_id].in([author.id])
    sub_query2 = Book.arel_table[:author_id].in(
      Author.arel_table.project(Author.arel_table[:id]))

    Book.includes([:chapters]).where(sub_query1).sum(:cost).should ==
      Book.includes([:chapters]).where(sub_query2).sum(:cost)

    # with sub_query1:
    #   SELECT SUM(`books`.`cost`) AS sum_id FROM `books`
    #     WHERE `books`.`author_id` IN (4)

    # with sub_query2:
    #   SELECT SUM(`books`.`cost`) AS sum_id FROM `books`
    #     LEFT OUTER JOIN `chapters` ON `chapters`.`book_id` = `books`.`id`
    #     WHERE `books`.`author_id` IN (SELECT `authors`.`id` FROM `authors`)
  end
end

Updated to match original repro correctly