campaigner.rb

module Crowdster
  module Models
    module Campaigner
      def included(klass)
        klass.class_eval do
          has_many :campaigns, class_name: 'Crowdster::Campaign'
        end
      end
    end
  end
end

user.rb

class User < ActiveRecord::Base
  include Crowdster::Models::Campaigner
end