#14518: Field.to_python not called on foreign key IDs
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Reporter: wolever | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: master
(models, ORM) | Resolution:
Severity: Normal | Triage Stage: Accepted
Keywords: | Needs documentation: 0
Has patch: 0 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Comment (by Harm Geerts <hgeerts@…>):
The reason the first example works is because in
Bar.objects.create(foo=m1) the m1 Foo instance is cached on the Bar
instance, so m2.foo.field does not perform a database query.
The second example fails because m2 has to perform a database lookup, and
that's where things get fun.
Your MyField.to_python is called multiple times so the value stored in the
database is actually "Value: Value: anubhavjoshi".
When you access bar.foo the primary key value "Value: Value: anubhavjoshi"
is passed through to_python again and becomes "Value: Value: Value:
anubhavjoshi" and that primary key does not exist.
The reason your patch works is because you only had one Foo record in your
dataset.
If you had multiple Foo records your test would raise
MultipleObjectsReturned.
The only thing the patch does is disable related lookup filters which is
not what you want ;)
It may be an idea to add a note to the documentation about custom field
classes.
--
Ticket URL: <https://code.djangoproject.com/ticket/14518#comment:11>
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