Creating basic association between ActiveRecord models and Mongoid documents

active_record_mongoid_bridge.rb

# Original source: http://hashrocket.com/blog/posts/bridging-activerecord-and-mongoid
class ActiveRecord::Base
  def self.has_many_documents(association_name)
    class_eval %<
      def #{association_name}
        #{association_name.to_s.singularize.classify}.where(#{name.underscore}_id: id)
      end
    >
  end

  def self.has_one_document(association_name)
    class_eval %<
      def #{association_name}
        #{association_name.to_s.singularize.classify}.where(#{name.underscore}_id: id).first
      end
    >

    class_eval %<
      def create_#{association_name} (new_attributes = {})
        #{association_name.to_s.singularize.classify}.create(new_attributes.merge(#{name.underscore}_id: id))
      end
    >
  end
end

module Mongoid::ActiveRecordBridge
  extend ActiveSupport::Concern

  included do
    def self.belongs_to_record(association_name, options={})
      association_class = options[:class_name] || association_name.to_s.singularize.classify
      class_eval %<
        field :#{association_name}_id, type: Integer
        index(#{association_name}_id: 1)

        def #{association_name}
          @#{association_name} ||= #{association_class}.where(id: #{association_name}_id).first if #{association_name}_id
        end

        def #{association_name}=(object)
          @#{association_name} = object
          self.#{association_name}_id = object.try :id
        end
      >
    end
  end
end