Add some comments about ID type choices

pgcommitfest/commitfest/models.py

@@ -375,7 +375,14 @@ class CfbotTask(models.Model):
         ('ERRORED', 'Errored'),
     ]
 
+    # This id is only used by Django. Using text type for primary keys, has
+    # historically caused problems.
     id = models.BigAutoField(primary_key=True)
+    # This is the id used by the external CI system. Currently with CirrusCI
+    # this is an integer, and thus we could probably store it as such. But
+    # given that we might need to change CI providers at some point, and that
+    # CI provider might use e.g. UUIDs, we prefer to consider the format of the
+    # ID opaque and store it as text.
     task_id = models.TextField(unique=True)
     task_name = models.TextField(null=False)
     patch = models.ForeignKey(Patch, on_delete=models.CASCADE, related_name="cfbot_tasks")