class User
  include Mongodid::Document

  embeds_one :address
end

class Address
  include Mongoid::Document

  embedded_in :user

  field :zip, type: Integer
end

user1 = User.create.build_address(zip: 3).save
user2 = User.create.build_address(zip: 5).save
user3 = User.create.build_address(zip: 1).save

This is correct sort:
User.desc(:"address.zip")
# => [user2, user1, user3]

But, Why is this wrong sort:
User.asc(:"address.zip")
# => [user2, user3, user1]