forms: fixed deletion of existing BlackBoardPush objects blackboard diffusion is removed

polynum_blackboard/forms.py

@@ -147,7 +147,7 @@ class MyCourseForm(request_forms.CopyrigtsForm):
             push.visible_to_students = self.cleaned_data['visible_to_students']
             push.save()
         else:
-            models.BlackBoardPush(request=instance).delete()
+            models.BlackBoardPush.objects.filter(request=instance).delete()
         return instance
 
     def pprint_data(self, data):