gistfile1.rb

############################################
# spec/spec_helper.rb, in RSpec.configure block

  config.before do
    MongoidCleaner.start
  end

  config.after do
    MongoidCleaner.clean
  end


############################################
# spec/support/mongoid.rb

class MongoidCleaner
  class <<self
    def start
      begin
        DatabaseCleaner.orm = 'mongoid'
        DatabaseCleaner.strategy = :truncation
        DatabaseCleaner.start
      ensure
        DatabaseCleaner.orm = 'active_record'
        DatabaseCleaner.strategy = :truncation
      end
    end


    def clean
      begin
        DatabaseCleaner.orm = 'mongoid'
        DatabaseCleaner.strategy = :truncation
        DatabaseCleaner.clean
      ensure
        DatabaseCleaner.orm = 'active_record'
        DatabaseCleaner.strategy = :truncation
      end
    end
  end
end



############################################
# lib/pseudo_attributes.rb

module PseudoAttributes
  extend ActiveSupport::Concern
  Null = (class NullClass; self; end).new

  included do
    alias_method_chain :reload, :pseudo_attributes if instance_methods.include?('reload')
    class_inheritable_accessor :_pseudos
    self._pseudos = {}
  end

  module InstanceMethods
    def reload_with_pseudo_attributes(*args)
      self.class.send(:_pseudos).each_key do |key| 
        instance_variable_set key, Null
      end
      reload_without_pseudo_attributes(*args)
    end
  end

  module ClassMethods
    def pseudo_attribute name, options={}, &block
      options.reverse_merge! :writer => false

      instance_variable = :"@#{name}"
      _pseudos[instance_variable] = block

      class_eval <<-RUBY, caller[1]
        def #{name}
          if !#{name}_populated?
            #{instance_variable} = instance_eval(&self.class.send(:_pseudos)[:#{instance_variable}]) 
          end

          #{instance_variable} == PseudoAttributes::Null ? nil : #{instance_variable}
        end

        def #{name}_populated?
          defined?(#{instance_variable}) && #{instance_variable} != PseudoAttributes::Null
        end
      RUBY

      if options[:writer]
        class_eval <<-RUBY
          def #{name}=(value)
            #{instance_variable} = value
          end
        RUBY
      end
    end
  end
end

########################################################
# Example use of PseudoAttributes

class User < ActiveRecord::Base
  ...
  pseudo_attribute :profiles do
    Profile.for_user(self)
  end
  ...
end