Wie filtere ich ForeignKey-Auswahlmöglichkeiten in einem Django ModelForm basierend auf der Muttergesellschaft?

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)

from models import *
from django.forms import ModelForm

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

    def __init__(self, company_id, *args, **kwargs):
        '''Filter the rate choices to only include those for the specified company'''
        super(ClientForm, self).__init__(*args, **kwargs)
        self.fields['base_rate'].queryset = Rate.objects.filter(company_id=company_id)

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,>