Rails engines extending functionality

I have an engine which defines some models and controllers. I want to be able to extend functionality of some models/controllers in my application (eg. adding methods) without loosing the original model/controller functionality from engine. Everywhere I read that you simply need to define controller with the same name in your application and Rails will automatically merge them, however it doesn't work for me and controller in engine is simply ignored (I don't think it's even loaded).

require MyEngine::Engine.root.join('app', 'models', 'my_engine', 'my_model')

before the model class definition in your application.

You can add these lines to you engine's module file in the lib root directory:

def self.root
  File.expand_path(File.dirname(File.dirname(__FILE__)))
end

def self.models_dir
  "#{root}/app/models"
end

def self.controllers_dir
  "#{root}/app/controllers"
end

Then you have the ability in the main application (the app making use of the engine) to require the necessary files from the engine. This is nice because you maintain Rails Engines default functionality and also have an easy tool for making use of normal ruby inheritance, without the need for patching.

EX:

#ENGINE Model -

class User < ActiveRecord::Base
  def testing_engine
    puts "Engine Method"  
  end
end

#MAIN APP Model -

require "#{MyEngine.models_dir}/user"
class User
  def testing_main_app
    puts "Main App Method"  
  end
end

#From the Main apps console

user = User.new

puts user.testing_engine #=>  "Engine Method"

puts user.tesing_main_app #=> "Main App Method"

Just if anyone else runs into same issue some time in the future, this is the code I wrote that fixed my problem:

module ActiveSupport::Dependencies
  alias_method :require_or_load_without_multiple, :require_or_load
  def require_or_load(file_name, const_path = nil)
    if file_name.starts_with?(RAILS_ROOT + '/app')
      relative_name = file_name.gsub(RAILS_ROOT, '')
      @engine_paths ||= Rails::Initializer.new(Rails.configuration).plugin_loader.engines.collect {|plugin| plugin.directory }
      @engine_paths.each do |path|
        engine_file = File.join(path, relative_name)
        require_or_load_without_multiple(engine_file, const_path) if File.file?(engine_file)
      end
    end
    require_or_load_without_multiple(file_name, const_path)
  end
end

This will automatically require files from engine before requiring from application if file path starts with 'app'.