#31295: required ModelChoiceField makes duplicate (cursor) queries to the
database
------------------------------------------------+------------------------
Reporter: Aurélien Pardon | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Forms | Version: 2.2
Severity: Normal | Keywords: Model
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
------------------------------------------------+------------------------
ModelChoiceField use ModelChoiceIterator for its {{{ queryset }}}/{{{
self.choices }}}, which use {{{.iterator()}}} and doesn't cache the query
under some conditions.
If the field is required, the method use_required_attribute
(https://github.com/django/django/blob/master/django/forms/widgets.py#L689)
fetch the first choice, making a duplicate query to the database (worse
than a useless query, the data may have changed):
{{{#!python
class Select(ChoiceWidget):
[...]
def use_required_attribute(self, initial):
[...]
first_choice = next(iter(self.choices), None)
}}}
Disabling the use of {{{.iterator()}}} (by adding an arbitrary
{{{.prefetch_related}}} for example) leads to no duplicate queries.
https://github.com/django/django/blob/da79ee472d803963dc3ea81ee67767dc06068aac/django/forms/models.py#L1152
:
{{{#!python
class ModelChoiceIterator:
[...]
def __iter__(self):
[...]
# Can't use iterator() when queryset uses prefetch_related()
if not queryset._prefetch_related_lookups:
queryset = queryset.iterator()
}}}
One solution would be to add another test to the previous piece of code :
(https://github.com/django/django/blob/da79ee472d803963dc3ea81ee67767dc06068aac/django/forms/models.py#L1152)
:
{{{#!python
if not queryset._prefetch_related_lookups and not
self.field.required:
queryset = queryset.iterator()
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/31295>
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/050.cd22376f80298f13eb8df90e08d5b5f7%40djangoproject.com.
