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
33950	ModelChoiceField and chained prefetch_related(): queries made twice	Vincent Lefoulon	nobody	"Hi!

I have these three models:

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

class Visit(models.Model):
    date = models.DateField()
    place = models.ForeignKey(Place, on_delete=models.CASCADE, related_name=""visits"")

class VisitDocument(models.Model):
    visit = models.ForeignKey(Visit, on_delete=models.CASCADE, related_name=""documents"")
    file = models.FileField()
}}}

A form to edit a visit:

{{{
class VisitForm(forms.ModelForm):
    class Meta:
        model = Visit
        exclude = (""place"",)

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        self.fields[""documents_to_delete""] = forms.ModelMultipleChoiceField(
            queryset=self.instance.documents.all(),
            required=False,
        )
}}}

And a generic view for the places:

{{{
class PlaceView(DetailView):
    queryset = Place.objects.prefetch_related(""visits__documents"")

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        if ""visit_form"" not in context:
            context[""visit_forms""] = [
                VisitForm(instance=visit)
                for visit in self.object.visits.all()
            ]

    def post(self, request, *args, **kwargs):
        // Handle the form
}}}

Despite the {{{prefetch_related(""visits__documents"")}}} call in my view, {{{ModelMultipleChoiceField()}}} doesn't detect that the documents of the visit (i.e. the form instance) are already fetched because {{{visit.documents.all()._prefetch_related_lookups}}} is null: https://github.com/django/django/blob/stable/3.2.x/django/forms/models.py#L1167

So is in the view: {{{self.object.visits.all()._prefetch_related_lookups}}} is null as well.

Many thanks!"	Bug	new	Uncategorized	3.2	Normal				Unreviewed	0	0	0	0	0	0