allow blank form input, use clean func to keep old val

todo/forms.py

@@ -49,6 +49,11 @@ class AddEditTaskForm(ModelForm):
 
     note = forms.CharField(widget=forms.Textarea(), required=False)
 
+    def clean_created_by(self):
+        """Keep the existing created_by regardless of anything coming from the submitted form.
+        If creating a new task, then created_by will be None, but we set it before saving."""
+        return self.instance.created_by
+
     class Meta:
         model = Task
         exclude = []