Filtering a drop-down list in Admin

[Chad]

Hi,

I am new to Django, I have read a good bit of the documentation and am 
trying to write an app. This app is simple and should allow for a lost of 
contacts and companies. Each company has different locations (or addresses) 
such as main office, branch office etc. Each contact should be assigned to 
only one company and one location. So that when looking up a contact the 
Name, Company, and Address can all be correct.

In the Django admin. I have company and location set-up with an Stacked 
inline so that the company name and all locations can be entered at once. 
Then I have a separate admin page for the contact. What I want to do is 
filter the drop-down list for the contact so that once a company is 
selected only the locations related to that company are displayed. I found 
this on filtering drop downs in admin 
http://www.stereoplex.com/blog/filtering-dropdown-lists-in-the-django-admin but 
I still cannot get it to work. The admin site works, it just lists all the 
Locations in the database weather a company is selected or not.

Here are my models:

from django.db import models

class Company(models.Model):
    company_name = models.CharField(max_length=200)
    website = models.URLField(verify_exists=False, max_length=200, 
blank=True)
    company_type = models.CharField(max_length=200, blank=True)
    company_trade = models.CharField(max_length=200, blank=True)
    company_notes = models.TextField(blank=True)
    custom = models.TextField(blank=True)
    def __unicode__(self):
        return self.company_name
    def is_subcontractor(self):
        return self.company_type == "subcontractor"

class Location(models.Model):
    description = models.CharField(max_length=200)
    company = models.ForeignKey(Company, related_name='location')
    address1 = models.CharField(max_length=200)
    address2 = models.CharField(max_length=200, blank=True)
    address3 = models.CharField(max_length=200, blank=True)
    city = models.CharField(max_length=200)
    state = models.CharField(max_length=2)
    zip_code = models.CharField(max_length=10)
    main_tel = models.CharField(max_length=200, blank=True)
    location_notes = models.TextField(blank=True)
    custom = models.TextField(blank=True)
    def __unicode__(self):
        return self.description

class Contact(models.Model):
    first_name = models.CharField(max_length=200)
    last_name = models.CharField(max_length=200)
    email = models.EmailField(max_length=75)
    company = models.ForeignKey(Company)
    location = models.ForeignKey(Location)
    title = models.CharField(max_length=200, blank=True)
    direct_tel = models.CharField(max_length=200, blank=True)
    mobile_tel = models.CharField(max_length=200, blank=True)
    contact_notes = models.TextField(blank=True)
    custom = models.TextField(blank=True)
    def __unicode__(self):
        return self.first_name + ' ' + self.last_name  

Here is admin.py:

 from contacts.models import Company,Location,Contact
from django.contrib import admin
from django.forms.models import BaseModelFormSet

class LocationInLine(admin.StackedInline):
    model = Location
    extra = 1
    fieldsets = [
        (None, {'fields': 
['description','address1','address2','address3','city','state','zip_code', 
'main_tel']}),
        ('Additional Notes', {'fields': ['location_notes', 'custom'], 
'classes': ['collapse']})
    ] 


class ContactLocationFormset(BaseModelFormSet):
    def add_fields(self, form, index):
        super(ContactLocationFormset, self).add_fields(form, index)
        locations = Location.objects.none()
        if form.instance:
            try:        
                company = form.instance.Company    
            except Company.DoesNotExist:
                pass   
            else:  
                locations = 
Locations.objects.filter(company=Location.company)
        form.fields['location'].queryset = locations


class ContactAdmin(admin.ModelAdmin):
    model = Contact
    formset = ContactLocationFormset
    fieldsets = [
        (None, {'fields': 
['first_name','last_name','email','company','location','title','direct_tel','mobile_tel']}),
        ('Additional Notes', {'fields': ['contact_notes', 'custom'], 
'classes': ['collapse']})
    ] 

class CompanyAdmin(admin.ModelAdmin):
    fieldsets = [
        (None, {'fields':   
 ['company_name','website','company_type','company_trade']}),
        ('Additional Notes', {'fields': ['company_notes', 'custom'], 
'classes': ['collapse']})
    ]        
    inlines = [LocationInLine]

admin.site.register(Company,CompanyAdmin)
admin.site.register(Contact,ContactAdmin)

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