acts_as_campaigner.rb

module Crowdster
  module ActsAsCampaigner
    def included(klass)
      klass.extend ClassMethods
    end

    module ClassMethods
      def acts_as_campaigner
        has_many :campaigns, class_name: 'Crowdster::Campaign'
      end
    end
  end
end

engine.rb

module Crowdster
  class Engine < ::Rails::Engine
    isolate_namespace Crowdster

    config.to_prepare do
      ActiveRecord::Base.include Crowdster::ActsAsCampaigner
    end
  end
end

user.rb

class User < ActiveRecord::Base
  acts_as_campaigner
end