added new test like defined in issue #712 and changed ObjectIdField to_python() method to use a try except similar to other Field classes

mongoengine/base/fields.py

@@ -410,8 +410,11 @@ class ObjectIdField(BaseField):
     """
 
     def to_python(self, value):
-        if not isinstance(value, ObjectId):
-            value = ObjectId(value)
+        try:
+            if not isinstance(value, ObjectId):
+                value = ObjectId(value)
+        except:
+            pass
         return value
 
     def to_mongo(self, value):

tests/queryset/queryset.py

@@ -4578,6 +4578,13 @@ class QuerySetTest(unittest.TestCase):
         self.assertEquals(Animal.objects(folded_ears=True).count(), 1)
         self.assertEquals(Animal.objects(whiskers_length=5.1).count(), 1)
 
+    def test_loop_via_invalid_id_does_not_crash(self):
+        class Person(Document):
+            name = StringField()
+        Person.objects.delete()
+        Person._get_collection().update({"name": "a"}, {"$set": {"_id": ""}}, upsert=True)
+        for p in Person.objects():
+            self.assertEqual(p.name, 'a')
 
 if __name__ == '__main__':
     unittest.main()