Make updated_{at,by} nullable

program/models.py

@@ -92,8 +92,8 @@ class ModelWithCreatedUpdatedFields(models.Model):
 
     created_at = models.DateTimeField(auto_now_add=True)
     created_by = models.CharField(max_length=150)
-    updated_at = models.DateTimeField(auto_now=True)
-    updated_by = models.CharField(max_length=150)
+    updated_at = models.DateTimeField(auto_now=True, blank=True, null=True)
+    updated_by = models.CharField(blank=True, max_length=150, null=True)
 
     class Meta:
         abstract = True