A Module to decoupling persistence from business model

persistence_mapper.rb

module Persistence
  module Mapper
    extend ActiveSupport::Concern

    included do
      extend ActiveModel::Naming
      include ActiveModel::Conversion
    end

    module ClassMethods

      def find(id)
        new.tap do |model|
          model.persistence = persistence_class.find(id)
        end
      end

      def method_missing(meth, *args, &blk)
        persistence_class.send meth, *args, &blk
      end

      def persistence_class
        @persistence_class ||= ("Persistence::#{name}".constantize rescue nil)
      end

      private
      def persistence_class=(klass)
        @persistence_class = (klass.is_a?(String) ? klass.constantize : klass)
        @persistence_class
      end

    end

    attr_accessor :persistence

    def initialize(obj = nil)
      raise "a persistence class is needed. Create one Persistence::#{self.to_model} or specify it with persistence_class= method." if self.class.persistence_class.nil?
      if obj.is_a? self.class.persistence_class
        @persistence = obj
      else
        @persistence = self.class.persistence_class.new obj
      end
      super
    end

    def create attrs
      attrs.each do |k,v|
        self.public_send "#{k}=", v
      end
      self.save!
    end

    def method_missing(meth, *args)
      unless @persistence
        @persistence = self.class.persistence_class.new
      end
      @persistence.send(meth, *args)
    end

  end

  #module Base
  #  include ActiveSupport::Concern
  #  def to_model(klass)
  #    klass
  #  end
  #end

end

persistence_mapper_spec.rb

require "spec_helper"
require_relative '../../lib/persistence_mapper'

describe Persistence::Mapper do

  class Persistence::Model; end

  subject do
    class Model
      include Persistence::Mapper
    end
    Model
  end

  let(:persistence_class) { stub(:persistence).as_null_object }

  context "ClassMethods" do
    context '#persistence_class' do

      it "should set automatically persistence to a Persistence class by default" do
        subject.persistence_class.should == Persistence::Model
      end

      it "should set automatically persistence to nil if Persistence Class doesn't exist" do
        class MyClass; include Persistence::Mapper; end
        MyClass.persistence_class.should be_nil
      end

    end

    context "#method_missing" do
      it "should delegate chain methods to persistence" do
        subject.send( :persistence_class=, persistence_class)
        persistence_class.should_receive(:unknown_method)
        subject.unknown_method
      end
    end

  end

  context 'InstanceMethods' do

    let(:object) { subject.new }
    let(:persistence) { stub(:persistence).as_null_object }
    before {
      subject.stub(:persistence_class).and_return(Persistence::Model)
    }

    it "should raise an error if has no persistence_class" do
      subject.stub(:persistence_class).and_return(nil)
      expect { subject.new }.should raise_error
    end

    it "should initialize with a new persistence object" do
      Persistence::Model.should_receive(:new)
      subject.new
    end

    it "should delegate methods to persistence object" do
      Persistence::Model.stub(:new).and_return(persistence)
      persistence.should_receive(:unknown_method)
      object.unknown_method
    end


  end

end

USAGE

#models/article.rb :

require 'persistence_mapper'
require_relative '../persistence/lotissement'
class Article
  include Persistence::Mapper
end

#persistence/article.rb :
module Persistence
  class Article < ActiveRecord::Base
    belongs_to :author
    validates_presence_of :title
  end
end