id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
34792	"Creating and saving a model using a custom primary key field can yield a bad ""id"" value on the instance"	mike w	nobody	"**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, [https://github.com/mik3y/django-spicy-id 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 [https://github.com/mik3y/django-spicy-id/blob/ce8cf54376f4cf0f82de9f385afaa70165eff2e3/django_spicy_id/fields.py#L147-L150 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 [https://github.com/django/django/blob/fbd16438f46bc2128926958ad24331da5d1b406f/django/db/models/sql/compiler.py#L1825-L1847 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 [https://github.com/django/django/blob/fbd16438f46bc2128926958ad24331da5d1b406f/django/db/models/base.py#L1057-L1063 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!


"	Bug	closed	Database layer (models, ORM)	4.2	Normal	duplicate	field, custom fields	Chris M	Unreviewed	0	0	0	0	0	0