Filter ManyToMany box in Django Admin

Question

I have a object with a many-to-many relation with another object.
In the Django Admin this results in a very long list in a multiple select box.

I'd like to filter the ManyToMany relation so I only fetch Categories that is available in the City that the Customer have selected.

Is this possible? Will I have to create a widget for it? And if so - how do I copy the behavior from the standard ManyToMany field to it, since I would like the filter_horizontal function as well.

These are my simplified models:

class City(models.Model):
    name = models.CharField(max_length=200)


class Category(models.Model):
    name = models.CharField(max_length=200)
    available_in = models.ManyToManyField(City)


class Customer(models.Model):
    name = models.CharField(max_length=200)
    city = models.ForeignKey(City)
    categories = models.ManyToManyField(Category)

Answer 1

Ok, this is my solution using above classes. I added a bunch more filters to filter it correctly, but I wanted to make the code readable here.

This is exactly what I was looking for, and I found my solution here: http://www.slideshare.net/lincolnloop/customizing-the-django-admin#stats-bottom (slide 50)

Add the following to my admin.py:

class CustomerForm(forms.ModelForm): 
    def __init__(self, *args, **kwargs):
        super(CustomerForm, self).__init__(*args, **kwargs)
        wtf = Category.objects.filter(pk=self.instance.cat_id);
        w = self.fields['categories'].widget
        choices = []
        for choice in wtf:
            choices.append((choice.id, choice.name))
        w.choices = choices


class CustomerAdmin(admin.ModelAdmin):
    list_per_page = 100
    ordering = ['submit_date',] # didnt have this one in the example, sorry
    search_fields = ['name', 'city',]
    filter_horizontal = ('categories',)
    form = CustomerForm

This filters the "categories" list without removing any functionality! (ie: i can still have my beloved filter_horizontal :))

The ModelForms is very powerful, I'm a bit surprised it's not covered more in the documentation/book.