gistfile1.rb

# Manage cross engine/plugin dependencies by deferring resolution 
# until all plugins have been loaded and then including them after 
# the main class definition has been loaded.

# lib/mercium.rb
module Mercium
  mattr_reader :extensions
  @@extensions = {}

  def self.register_extension(klass, extension)
    extensions[klass] = Array(extensions[klass]) << extension
  end

  def self.load_extensions(klass)
    Array(extensions[klass]).each do |extension|
      require_dependency extension
      klass.to_s.camelize.constantize.send :include, extension.camelize.constantize
    end
  end
end


# app/models/user.rb
class User < ActiveRecord::Base
end
Mercium.load_extensions :user


# vendor/plugins/mercium_blog/init.rb
Mercium.register_extension :user, 'user/blogs'


# vendor/plugins/mercium_blog/app/models/user/blogs.rb
class User < ActiveRecord::Base
  module Blogs
    def self.included(base)
      base.class_eval do
        has_many :blogs
      end
    end
  end
end

# vendor/plugins/mercium_blog/app/models/blog.rb
class Blog < ActiveRecord::Base
  belongs_to :user
end
Mercium.load_extensions :blog