Personally I don't feel that it's a particularly common need, but I can see your use case. More common is that a reasonable filter is applicable - say books with a rating over 80.
As far as I know we don't have any support for SQL intersects and I don't believe there are any plans to introduce it. I do agree that this should be both documented and preferably raise an error - please open a ticket. As for a work around, using a ChoiceField directly shouldn't be particularly difficult in this case. ModelChoiceField has some nice features but it is limited. Marc On 21 Oct 2014 17:42, "Kamil Śliwak" <[email protected]> wrote: > Hi > > I have just stumbled upon a problem with how ModelChoiceField handles its > 'queryset' argument and I'd like to ask whether it's a deliberate design > choice or if I should report it as a bug. > > > Let's say you have a model called Book: > > class Book(models.Model): > rating = models.IntegerField() > > You want to create a form that lets user select one of tghe top rated > books. So you try: > > class BookForm(forms.Form): > book = forms.ModelChoiceField(queryset = > Book.objects.order_by('-rating')[:100]) > > It appears to work - the form can be rendered and you can choose one of a > hundered top-rated books. But when you submit, you get the following error: > > AssertionError: Cannot filter a query once a slice has been taken. >> > > The error is caused by ModelChoiceField.to_python() > <https://github.com/django/django/blob/master/django/forms/models.py#L1186-L1194> > validating the existence of the selected item by calling get() on the > queryset: > > value = self.queryset.get(**{key: value}) > > And this is not supported for sliced querysets as the error above states. > It might have been be a deliberate choice not to support this use case in > ModelChoiceField though I don't see any comments about that in the code or > any mentions in ModelChoiceField.queryset > <https://docs.djangoproject.com/en/1.6/ref/forms/fields/#django.forms.ModelChoiceField.queryset> > docs. I would also expect it to be a common need. This and the fact that it > can be trivially solved by replacing get() with SQL INTERSECT (implemented > as operator & in Django) makes me suspect that there would be some issue > with that. > > Can anyone shed some light on this? Are there any performance or > database-combatibility issues involved? Or is it just a bug? > > -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > Visit this group at http://groups.google.com/group/django-developers. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-developers/e264b254-4208-4e39-a50b-6657f8817996%40googlegroups.com > <https://groups.google.com/d/msgid/django-developers/e264b254-4208-4e39-a50b-6657f8817996%40googlegroups.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/CAMwjO1H_Kt101T8-Qd7nJ%2BMFVbR0voQVaHGifUUiQGFWGiwi2A%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
