Added to_dbref

Thanks to Ankhbayar for the initial code
Closes #202

mongoengine/document.py

@@ -167,6 +167,14 @@ class Document(BaseDocument):
             value._changed_fields = []
         return value
 
+    def to_dbref(self):
+        """Returns an instance of :class:`~pymongo.dbref.DBRef` useful in
+        `__raw__` queries."""
+        if not self.pk:
+            msg = "Only saved documents can have a valid dbref"
+            raise OperationError(msg)
+        return pymongo.dbref.DBRef(self.__class__._meta['collection'], self.pk)
+
     @classmethod
     def register_delete_rule(cls, document_cls, field_name, rule):
         """This method registers the delete rules to apply when removing this