#33950: ModelChoiceField and chained prefetch_related(): queries made twice
--------------------------------------------+------------------------
Reporter: Vincent Lefoulon | Owner: nobody
Type: Bug | Status: new
Component: Uncategorized | Version: 3.2
Severity: Normal | Keywords:
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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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!
--
Ticket URL: <https://code.djangoproject.com/ticket/33950>
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