#27710: foreign key _id optimisation leads to incorrect descriptor value
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Reporter: Will Hardy | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 1.10
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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A common optimisation (also recommended in the docs:
https://docs.djangoproject.com/en/1.8/topics/db/optimization/#use-foreign-
key-values-directly) is to use `MyModel.relatedfield_id = 5` for setting a
foreign key, when you want to avoid querying the database.
However, if the instance is then later used (accidentally or occasionally,
that would defeat the point of the optimisation), the value of
`MyModel.relatedfield` can be incorrect. I think inefficient would be a
better default than incorrect, because the mistake is subtle.
I imagine the `ForwardManyToOneDescriptor` could check the primary key of
the cache value being returned against the primary key on the model
instance and if there is a mismatch, then the cache value should be
deleted and a new query should be made.
{{{
>>> instance = MyModel(relatedfield_id=1)
>>> print(instance.relatedfield.pk)
1
>>> instance.relatedfield_id = 2
>>> print(instance.relatedfield.pk)
1
>>> instance.save()
>>> print(instance.relatedfield.pk)
1
}}}
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Ticket URL: <https://code.djangoproject.com/ticket/27710>
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