You’re not limited to the functionality that Rails automatically builds into association proxy objects. You can also extend these objects through anonymous modules, adding new finders, creators, or other methods. For example:

Association Extensions.rb

class Customer < ActiveRecord::Base
  has_many :orders do
    def find_by_order_prefix(order_number)
      find_by_region_id(order_number[0..2])
    end
  end
end