#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 | Resolution: Keywords: | Triage Stage: Unreviewed Has patch: 0 | Needs documentation: 0 Needs tests: 0 | Patch needs improvement: 0 Easy pickings: 0 | UI/UX: 0 ----------------------------------+-------------------------------------- Description changed by Vincent Lefoulon:
Old description: > 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! New description: 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_forms" 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. Then the queries are made twice, erasing the {{{prefetch_related()}}} effect. Many thanks! -- -- Ticket URL: <https://code.djangoproject.com/ticket/33950#comment:2> 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 django-updates+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/django-updates/01070182ca2e9a00-ab9ef0d8-815d-45a7-8fa9-2aac2d514aa8-000000%40eu-central-1.amazonses.com.