Filter ManyToMany box in Django Admin

Question

I have an object with a ManyToMany 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 are available in the City that the Customer has 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.