Resolve "re-submit fails with label only"

core/serializers/feedback.py

@@ -160,11 +160,16 @@ class FeedbackSerializer(DynamicFieldsModelSerializer):
                               self.instance is not None and
                               self.instance.feedback_lines.count() > 0)
 
-        has_label_attached = 'labels' in data and len(data['labels']) > 0
+        has_label_attached = ('labels' in data and len(data['labels']) > 0 or
+                              self.instance is not None and
+                              self.instance.labels.count() > 0)
+
+        labels_get_deleted = 'labels' in data and len(data['labels']) == 0
 
         # a non-full scored feedback is considered valid if there is
         # at least one comment line or a label attached to the feedback
-        if not (has_full_score or has_feedback_lines or has_label_attached):
+        if not (has_full_score or has_feedback_lines or
+                (has_label_attached and not labels_get_deleted)):
             raise serializers.ValidationError(
                 'Sorry, you have to explain why this does not get full score')