has_many_documents.rb

# source: http://hashrocket.com/blog/posts/bridging-activerecord-and-mongoid
module Extensions::ActiveRecord::HasManyDocuments
  extend ActiveSupport::Concern

  module ClassMethods
    def has_many_documents(association_name, options = {})
      class_eval %<
        def #{association_name}
          #{association_name.to_s.singularize.classify}.where(#{name.underscore}_id: id)
        end
      >
    end
  end
end

ActiveRecord::Base.send(:include, Extensions::ActiveRecord::HasManyDocuments)