model-mixin-example.rb

module Sold

  def self.included(base)
    base.extend ClassMethods
    base.instance_eval do 
      belongs_to :seller, :polymorphic => true
      include InstanceMethods
    end
  end
  
  module ClassMethods
    # seller-related class methods
  end
  
  module InstanceMethods
    # seller-related instance methods
  end

end

TheModel.send(:include, Sold)