From dc5a613bc7a28148791f8fa42a7cebfe83fe5a5f Mon Sep 17 00:00:00 2001
From: Ross Lawley <ross.lawley@gmail.com>
Date: Mon, 11 Jul 2011 09:19:27 +0100
Subject: [PATCH] Fixes conversion of null genericreferences in querysets

closes #211
---
 mongoengine/fields.py |  3 +++
 tests/fields.py       | 12 ++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index eb0825e6..88040115 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -653,6 +653,9 @@ class GenericReferenceField(BaseField):
         return doc
 
     def to_mongo(self, document):
+        if document is None:
+            return None
+
         id_field_name = document.__class__._meta['id_field']
         id_field = document.__class__._fields[id_field_name]
 
diff --git a/tests/fields.py b/tests/fields.py
index f2543fc1..7a752998 100644
--- a/tests/fields.py
+++ b/tests/fields.py
@@ -1095,6 +1095,18 @@ class FieldTest(unittest.TestCase):
         Link.drop_collection()
         User.drop_collection()
 
+    def test_generic_reference_is_none(self):
+
+        class Person(Document):
+            name = StringField()
+            city = GenericReferenceField()
+
+        Person.drop_collection()
+        Person(name="Wilson Jr").save()
+
+        self.assertEquals(repr(Person.objects(city=None)),
+                            "[<Person: Person object>]")
+
     def test_binary_fields(self):
         """Ensure that binary fields can be stored and retrieved.
         """