#31295: required ModelChoiceField makes duplicate (cursor) queries to the
database
-------------------------------------+-------------------------------------
Reporter: Aurélien Pardon | Owner: nobody
Type: | Status: closed
Cleanup/optimization |
Component: Forms | Version: 2.2
Severity: Normal | Resolution: needsinfo
Keywords: Model | 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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Comment (by Aurélien Pardon):
Replying to [comment:4 Carlton Gibson]:
> (Yes there's an extra query but correctness trumps performance.)
How is that correct if the queries yield different results ?
It only works if the two requests are inside a transaction, for example
when {{{ATOMIC_REQUESTS}}} is set to {{{True}}} (which is not the
default).
For example, imagine this model (the primary key is important, it's the
value that the choice will use):
{{{#!python
class MyModel(models.Model):
my_field = models.CharField(max_length=1, primary_key=True)
}}}
If the first query (for the actual choices) returns {{{['a', 'b']}}}, but
the second one (for the required attribute) returns {{{['', 'b', 'c']}}},
then the required attribute will be used leading to invalid HTML.
--
Ticket URL: <https://code.djangoproject.com/ticket/31295#comment:5>
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