Demonstrating a Rails 4.0.4 bug.

order_direction_bug.rb

# Tested on Rails 4.0.4
# Rails 4.1.0.rc2 has fixed it.
require 'active_record'

ActiveRecord::Base.establish_connection adapter: "sqlite3", database: ":memory:"

ActiveRecord::Migration.create_table :library

ActiveRecord::Migration.create_table :books do |t|
  t.string :name
  t.integer :library_id
end

class Library < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  belongs_to :libraries
end

puts Book.order(:name).to_sql
# SELECT "books".* FROM "books"   ORDER BY "books"."name" ASC -> Great! √

puts Library.joins(:books).merge(Book.order(:name)).to_sql
# SELECT "libraries".* FROM "libraries" INNER JOIN "books" ON "books"."library_id" = "libraries"."id"  ORDER BY "books"."name" ASC -> Great! √

puts Book.order(name: :desc).to_sql
# SELECT "books".* FROM "books"   ORDER BY "books"."name" DESC -> Great! √

#
# The problem:
#
puts Library.joins(:books).merge(Book.order(name: :desc)).to_sql
# Expected:
# SELECT "libraries".* FROM "libraries" INNER JOIN "books" ON "books"."library_id" = "libraries"."id"  ORDER BY "books"."name" DESC

# Returned:
# SELECT "libraries".* FROM "libraries" INNER JOIN "books" ON "books"."library_id" = "libraries"."id"  ORDER BY "libraries"."name" DESC