Fixed issue with unset operation

mongoengine/queryset.py

@@ -917,8 +917,7 @@ class QuerySet(object):
 
                 # Convert value to proper value
                 field = fields[-1]
-                if op in (None, 'set', 'unset', 'pop', 'push', 'pull',
+                if op in (None, 'set', 'push', 'pull', 'addToSet'):
-                          'addToSet'):
                     value = field.prepare_query_value(op, value)
                 elif op in ('pushAll', 'pullAll'):
                     value = [field.prepare_query_value(op, v) for v in value]

tests/queryset.py

@@ -1462,6 +1462,27 @@ class QuerySetTest(unittest.TestCase):
 
         Number.drop_collection()
 
+    def test_unset_reference(self):
+        class Comment(Document):
+            text = StringField()
+
+        class Post(Document):
+            comment = ReferenceField(Comment)
+
+        Comment.drop_collection()
+        Post.drop_collection()
+
+        comment = Comment.objects.create(text='test')
+        post = Post.objects.create(comment=comment)
+
+        self.assertEqual(post.comment, comment)
+        Post.objects.update(unset__comment=1)
+        post.reload()
+        self.assertEqual(post.comment, None)
+
+        Comment.drop_collection()
+        Post.drop_collection()
+
 
 class QTest(unittest.TestCase):