Pattern for extending ActiveRecord models

admin.rb

# app/models/user_extensions/admin.rb

module UserExtensions
  module Admin
    extend ActiveSupport::Concern

    ADMIN = 'admin'

    module ClassMethods

      def acts_as_admin(options = {})
        attr_accessor :admin_password_confirmation
        validates_presence_of :admin_password
      end

      # more fat user admin class methods
    end

    module InstanceMethods
      def admin?
        role == ADMIN
      end

      # more fat user admin instance methods
    end

  end
end

gravatar-user.rb

# app/models/gravatar/user.rb

module Gravatar

  class User < ::User
    include UserExtensions::Gravatar

    default_scope select(%w[ id name gravatar_id ])
    
    # additional slim user instance methods
  end

end

user.rb

# app/models/user.rb

class User < ActiveRecord::Base

  include UserExtensions::Admin
  include UserExtensions::Memberships

  // lots more fat code
end