id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
18597	`BaseInlineFormSet` should attempt to get it's queryset from it's instance related manager before falling back to it's model's default manager	Simon Charette	nobody	"The newly introduced `prefetch_related` method can be quite handy to avoid unnecessary queries (thanks Luke :), however it's quite useless when used with `BaseInlineFormSet` since [https://github.com/django/django/blob/4a103086d5c67fa4fcc53c106c9fdf644c742dd8/django/forms/models.py#L694 it won't even try to get its needed `queryset` from the related manager].

i.e.
{{{#!python
from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=255)

class Book(models.Model):
    name = models.CharField(max_length=255)
    author = models.ForeignKey(Author, related_name='books')
    
    class Meta:
        # Needed because `BaseModelFormSet.get_query_set` wants an ordered set or issue another query
        ordering = ('pk',)
}}}

{{{
In [1]: from django.conf import settings

In [2]: from django.db import connection

In [3]: from library.models import *

In [4]: from django.forms.models import inlineformset_factory

In [5]: a = Author.objects.create(name='David Abram')

In [6]: b1 = Book.objects.create(name='Becoming Animal', author=a)

In [7]: b2 = Book.objects.create(name='The Spell of the Sensuous', author=a)

In [8]: BookInlineFormSet = inlineformset_factory(Author, Book)

In [9]: settings.DEBUG = True

In [10]: instance = Author.objects.prefetch_related('books').get()

In [11]: BookInlineFormSet(instance=instance)
Out[11]: <django.forms.formsets.BookFormFormSet at 0x3c68d50>

In [12]: print connection.queries
[{u'time': u'0.000', u'sql': u'SELECT ""library_author"".""id"", ""library_author"".""name"" FROM ""library_author""'}, {u'time': u'0.000', u'sql': u'SELECT ""library_book"".""id"", ""library_book"".""name"", ""library_book"".""author_id"" FROM ""library_book"" WHERE ""library_book"".""author_id"" IN (1) ORDER BY ""library_book"".""id"" ASC'},
 {u'time': u'0.000', u'sql': u'SELECT ""library_book"".""id"", ""library_book"".""name"", ""library_book"".""author_id"" FROM ""library_book"" WHERE ""library_book"".""author_id"" = 1  ORDER BY ""library_book"".""id"" ASC'}]
}}}

I guess it's only a matter of time before people start trying to optimize their formsets (or their `ModelAdmin` by overriding `get_queryset`) and stumble on this ''limitation''.

I've written a patch (I'll create a pull request) for which I'll write tests if this optimization is ''Accepted''."	Cleanup/optimization	new	Forms	1.4	Normal		model formset inline		Unreviewed	0	0		0	0	0