ModelChoiceField and chained prefetch_related(): queries made twice

[Vincent Lefoulon]

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.fieldsdocuments_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("visitsdocuments")

def get_context_data(self, kwargs):

context = super().get_context_data(kwargs)
if "visit_form" not in context:

contextvisit_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!