How do I filter ForeignKey choices in a Django ModelForm?

Question

Say I have the following in my models.py:

class Company(models.Model):
   name = ...

class Rate(models.Model):
   company = models.ForeignKey(Company)
   name = ...

class Client(models.Model):
   name = ...
   company = models.ForeignKey(Company)
   base_rate = models.ForeignKey(Rate)

I.e. there are multiple Companies, each having a range of Rates and Clients. Each Client should have a base Rate that is chosen from it's parent Company's Rates, not another Company's Rates.

When creating a form for adding a Client, I would like to remove the Company choices (as that has already been selected via an "Add Client" button on the Company page) and limit the Rate choices to that Company as well.

How do I go about this in Django 1.0?

My current forms.py file is just boilerplate at the moment:

from models import *
from django.forms import ModelForm

class ClientForm(ModelForm):
    class Meta:
        model = Client

And the views.py is also basic:

from django.shortcuts import render_to_response, get_object_or_404
from models import *
from forms import *

def addclient(request, company_id):
    the_company = get_object_or_404(Company, id=company_id)

    if request.POST:
        form = ClientForm(request.POST)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect(the_company.get_clients_url())
    else:
        form = ClientForm()

    return render_to_response('addclient.html', {'form': form, 'the_company':the_company})

In Django 0.96 I was able to hack this in by doing something like the following before rendering the template:

manipulator.fields[0].choices = [(r.id,r.name) for r in Rate.objects.filter(company_id=the_company.id)]

ForeignKey.limit_choices_to seems promising but I don't know how to pass in the_company.id and I'm not clear if that will work outside the Admin interface anyway.

Thanks. (This seems like a pretty basic request but if I should redesign something I'm open to suggestions.)

Answer 1

ForeignKey is represented by django.forms.ModelChoiceField, which is a ChoiceField whose choices are a model QuerySet. See the reference for ModelChoiceField.

So, provide a QuerySet to the field's queryset attribute. Depends on how your form is built. If you build an explicit form, you'll have fields named directly.

form.rate.queryset = Rate.objects.filter(company_id=the_company.id)

If you take the default ModelForm object, form.fields["rate"].queryset = ...

This is done explicitly in the view. No hacking around.