mutation.rb

module Mongoid
  
  module Mutation
    
    def as(klass, extra_attributes={})
      klass.new(attributes_for(klass, extra_attributes)).tap do |mutation|
        mutation.new_record = new_record?
        mutation.send(:modifications)['_type'] = [self._type, klass.name]
      end
    end
    
    def as!(klass, extra_attributes={})
      new_attributes = Hash[attributes_for(klass, extra_attributes, false).map{ |k, v| [k, [nil, v]]}].merge({'_type' => [self._type, klass.name]})
      send(:modifications).update(new_attributes)
      save!
      klass.find(self._id)
    end
    
    private
    
    def attributes_for(klass, extra_attributes, new_record=true)
      shared_fields = klass.fields.keys
      shared_fields << '_id' if new_record
      shared_attributes = self.attributes.slice(*shared_fields)
      shared_attributes.merge(extra_attributes)
    end
    
  end
  
end