Opened 15 months ago
Last modified 15 months ago
#34792 closed Bug
Creating and saving a model using a custom primary key field can yield a bad "id" value on the instance — at Initial Version
Reported by: | mike w | Owned by: | nobody |
---|---|---|---|
Component: | Database layer (models, ORM) | Version: | 4.2 |
Severity: | Normal | Keywords: | field, custom fields |
Cc: | Chris M | Triage Stage: | Unreviewed |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
Summary: In some situations, creating and saving a model which uses a custom primary key can cause a wrong ".id" value to be set on the instance in memory.
Detail: I am using a custom primary key Field type which inherits from AutoField, django-spicy-id. Essentially what this Field does is translate the underlying integer data in the database, to and from a string (with a static prefix) at the Django level.
It does this by implementing from_db_value() and friends in the Field implementation.
This might not be to everyone's taste, but so far I think it is completely within the acceptable functionality & contract of custom Fields.
I noticed that in some environments, particularly in Django pre-4.0, save()
on a new instance would cause the instance's .id
field to be the underlying number - and not the expected string value, which the custom Field implements. Example:
class MyModel(models.Model): id = SpicyBigAutoField("ex", primary_key=True, randomize=True)
>>> o = MyModel() >>> o.save() >> o.id 3735928559 # yikes! we should never get a number here. >>> o.refresh_from_db() >>> o.id 'ex_deadbeef`
I believe the culprit is this block in django.db.models.sql.compiler, specifically the last condition where self.connection.ops.last_insert_id is called and returned without modification.
In turn, the parent caller django.db.models.base.Model::_save_table() will blindly setattr this value on the instance.
It seems like a possible fix could be to flow the "last_insert_id" case through the Field's from_db_value(); in most cases this is surely a no-op, but.. not in this case.
(I'd happy to prepare a patch and/or test if offered a little guidance.) Thanks!