Django - Limit choices to something that depends o

I have some foos which are organized into categories. For each category, I want to be able to select a winner foo.

Hence I have models which look like this:

class Category(models.Model):
    name = models.CharField(max_length=30)
    # More fields...
    winner = models.ManyToManyField(
        'Foo',
        related_name='winner'
    )

class Foo(models.Model):
    name = models.CharField(max_length=30)
    # More fields...
    category = models.ForeignKey(
        Category,
        related_name='category'
    )

(The reason why winner is a ManyToManyField is that a single foo may belong to several categories, while in a single category there may be more than one winner due to ex-aequo.)

I want to impose the natural constraint that a foo can win in a category only if it belongs to that category. The most reasonable way to do so seems to use the limit_choices_to parameter, but it seems to me that it is not possible to limit the choices based on the current instance of the model.

I may be able to impose this constraint in a given form, but I would like to avoid this for two reasons:

the constraint naturally lives at the model level. It is a particular relation that should always hold between two related models
the choice of the winner will be made in the admin, and I would like to avoid having to customize the admin forms

Is there any way to impose this constraint in Django?

There is no way to put constraint on M2M field on the models level (ie with limit_choices_to). However, you can do this in the form:

class MyForm(forms.ModelForm):
    class Meta:
        model = models.Category

    def __init__(self, *args, **kwargs):
        super(MyForm, self).__init__(*args, **kwargs)
        if 'instance' in kwargs:
            my_category = kwargs['instance']
            self.fields['winner'].queryset = my_category.category.all()

Well, did you notice that?

my_category.category.all()

Probably what you want is:

class Foo(models.Model):
    name = models.CharField(max_length=30)
    # More fields...
    category = models.ForeignKey(
        Category,
        related_name='participants'
    )