ActiveModularity

gem 'active_modularity'

$ bundle

$ gem install active_modularity

# config/initializers/acts_as_modurarity.rb

# enable active modurality
ActiveRecord::Base.acts_as_modurality

# load all files (Must be loaded the models and controllers.)
YourApp::Application.eager_load! unless Rails.configuration.cache_classes

#
# has many association
# 

# app/models/user.rb
class Person < ActiveRecord::Base
  has_many :entries
end

# app/models/entry.rb
class Entry < ActiveRecord::Base
  belongs_to :person
end

# app/models/admin/user.rb
module Admin
  class Person < ::Person
  end
end

# app/models/admin/user.rb
module Admin
  class Entry < ::Entry
  end
end

# standard association
person = Person.create
person.entries.build.class.name # Entry

# inner module association
admin_person = Admin::Person.create
admin_person.entries.build.class.name # Admin::Entry


#
# single table inheritance
#

# app/models/person.rb
class Person < ActiveRecord::Base
end

# app/models/customer.rb
class Customer < Person
end

# app/models/employee.rb
class Employee < Person
end

# app/models/admin/person.rb
module Admin
  class Person < ::Person
    module Common
      extend ActiveSupport::Concern
      included do
        # Admin::Person common class macro
        validates :name, :presence => true
      end
      
      def common_method
        # do something...
      end
    end
  end
end

# app/models/admin/customer.rb
module Admin
  class Customer < ::Customer
    include Admin::Person::Common
  end
end

# app/models/admin/employee.rb
module Admin
  class Employee < ::Employee
    include Admin::Person::Common
  end
end


# create
alice = Customer.create(:name => "alice")
alice.class.name # Customer
alice.type       # Customer

bob = Admin::Customer.create(:name => "bob")
bob.class.name   # Admin::Customer
bob.type         # Customer (not Admin::Customer)

# find 
Person.find(alice.id).class.name        # Customer
Person.find(bob.id  ).class.name        # Customer
Admin::Person.find(alice.id).class.name # Admin::Customer
Admin::Person.find(bob.id  ).class.name # Admin::Customer