model with namespace inheritance

I'd like to extract custom gem with shared models. This models will be shared between several applications and extended if needed. For example i have shared model like:

module CommonModels
  class Customer < ActiveRecord::Base
    has_many :orders
  end
end

module CommonModels
  class Order < ActiveRecord::Base
    has_one :customer
  end
end

And in app models extends common:

class Customer < CommonModels::Customer
  validates :name, presence: true
end

class Order < CommonModels::Order
  validates :price, presence: true
end

Inheritance works well but when i try to get customer order or build new object from relation instead of Order i will get CommonModel::Order. Is there a way to return extended Order class instead of parent (like some kind of polimorphism)?

I do not like to specify relations in each dependent model but i want to use all features from specified version without 'casting' to child object.