Fixed reload issue with ReferenceField where dbref=False (MongoEngine/mongoengine#138)

mongoengine/document.py

@@ -361,7 +361,12 @@ class Document(BaseDocument):
         id_field = self._meta['id_field']
         obj = self.__class__.objects(
                 **{id_field: self[id_field]}
-              ).first().select_related(max_depth=max_depth)
+              ).limit(1).select_related(max_depth=max_depth)
+        if obj:
+            obj = obj[0]
+        else:
+            msg = "Reloaded document has been deleted"
+            raise OperationError(msg)
         for field in self._fields:
             setattr(self, field, self._reload(field, obj[field]))
         if self._dynamic: