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 = []

todo/migrations/0011_auto_20190724_1130.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.1.8 on 2019-07-24 11:30
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('todo', '0010_attachment'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='task',
+            name='created_by',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='todo_created_by', to=settings.AUTH_USER_MODEL),
+        ),
+    ]

todo/models.py

@@ -78,9 +78,9 @@ class Task(models.Model):
     created_by = models.ForeignKey(
         settings.AUTH_USER_MODEL,
         null=True,
+        blank=True,
         related_name="todo_created_by",
         on_delete=models.CASCADE,
-        editable=False,
     )
     assigned_to = models.ForeignKey(
         settings.AUTH_USER_MODEL,