#34792: Creating and saving a model using a custom primary key field can yield a
bad "id" value on the instance
-------------------------------------+-------------------------------------
Reporter: mike w | Owner: nobody
Type: | Status: new
Uncategorized |
Component: Database | Version: 4.2
layer (models, ORM) | Keywords: field, custom
Severity: Normal | fields
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
**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!
--
Ticket URL: <https://code.djangoproject.com/ticket/34792>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/0107018a1f17984f-747bf5f3-27a1-402d-addf-820ff4154a22-000000%40eu-central-1.amazonses.com.
