Added test and fix for delete with write_concern w:0

docs/changelog.rst

@@ -26,6 +26,7 @@ Changes in 0.9.X - DEV
 - Removed get_or_create() deprecated since 0.8.0. #300
 - Capped collection multiple of 256. #1011
 - Added `BaseQuerySet.aggregate_sum` and `BaseQuerySet.aggregate_average` methods.
+- Fix for delete with write_concern {'w': 0}. #1008
 
 Changes in 0.9.0
 ================

mongoengine/queryset/base.py

@@ -418,7 +418,8 @@ class BaseQuerySet(object):
                     **{'pull_all__%s' % field_name: self})
 
         result = queryset._collection.remove(queryset._query, **write_concern)
-        return result["n"]
+        if result:
+            return result.get("n")
 
     def update(self, upsert=False, multi=True, write_concern=None,
                full_result=False, **update):

tests/queryset/queryset.py

@@ -1634,6 +1634,11 @@ class QuerySetTest(unittest.TestCase):
         self.Person.objects()[:1].delete()
         self.assertEqual(1, BlogPost.objects.count())
 
+    def test_limit_with_write_concern_0(self):
+
+        p1 = self.Person(name="User Z", age=20).save()
+        p1.delete(w=0)
+
     def test_reference_field_find(self):
         """Ensure cascading deletion of referring documents from the database.
         """