[Solved]-Country/State/City dropdown menus inside the Django admin inline

With a little hackery, it’s quite doable.

In the following example, County is used instead of State and Municipality instead of City. So, the models are as follows:

class County(models.Model):
    name = models.CharField(_('Name'), max_length=100, unique=True)

class Municipality(models.Model):
    county = models.ForeignKey(County, verbose_name=_('County'))
    name = models.CharField(_('Name'), max_length=100)

class Location(models.Model):
    name = models.CharField(max_length=100)
    county = models.ForeignKey(County, verbose_name=_('County'))
    municipality = models.ForeignKey(Municipality,
            verbose_name=_("Municipality"))

There are two sides of the problem: client-side JavaScript and server side field rendering.

The client side JavaScript (with JQuery, assumed to be served from /site_media/js/municipality.js) is as follows:

var response_cache = {};

function fill_municipalities(county_id) {
  if (response_cache[county_id]) {
    $("#id_municipality").html(response_cache[county_id]);
  } else {
    $.getJSON("/municipalities_for_county/", {county_id: county_id},
      function(ret, textStatus) {
        var options = '<option value="" selected="selected">---------</option>';
        for (var i in ret) {
          options += '<option value="' + ret[i].id + '">'
            + ret[i].name + '</option>';
        }
        response_cache[county_id] = options;
        $("#id_municipality").html(options);
      });
  }
}

$(document).ready(function() {
  $("#id_county").change(function() { fill_municipalities($(this).val()); });
});

Now you need the Ajax view for serving municipalities that belong to a given county (assumed to be served from /municipalities_for_county/):

from django.http import JSONResponse
from django.utils.encoding import smart_unicode
from django.utils import simplejson

from myproject.places.models import Municipality

def municipalities_for_county(request):
    if request.is_ajax() and request.GET and 'county_id' in request.GET:
        objs = Municipality.objects.filter(county=request.GET['county_id'])
        return JSONResponse([{'id': o.id, 'name': smart_unicode(o)}
            for o in objs])
    else:
        return JSONResponse({'error': 'Not Ajax or no GET'})

And finally the server side code in admin.py for rendering the field is as follows.
First, the imports:

from django import forms
from django.forms import widgets
from django.forms.utils import flatatt
from django.utils.encoding import smart_text
from django.utils.safestring import mark_safe
from django.contrib import admin
from django.utils.translation import gettext_lazy

from myproject.places.models import Municipality, Location

Then, the widget:

class MunicipalityChoiceWidget(widgets.Select):
    def render(self, name, value, attrs=None, choices=()):
        self.choices = [(u"", u"---------")]
        if value is None:
            # if no municipality has been previously selected,
            # render either an empty list or, if a county has
            # been selected, render its municipalities
            value = ''
            model_obj = self.form_instance.instance
            if model_obj and model_obj.county_id:
                for m in model_obj.county.municipality_set.all():
                    self.choices.append((m.id, smart_text(m)))
        else:
            # if a municipality X has been selected,
            # render only these municipalities, that belong
            # to X's county
            obj = Municipality.objects.get(id=value)
            for m in Municipality.objects.filter(county=obj.county):
                self.choices.append((m.id, smart_text(m)))

        s = widgets.Select(choices=self.choices)
        select_html = s.render(name=name,value=value,attrs=attrs)

        return mark_safe(''.join(select_html))

Next, the form:

class LocationForm(forms.ModelForm):
    municipality = forms.ModelChoiceField(Municipality.objects,
            widget=MunicipalityChoiceWidget(),
            label=gettext_lazy("Municipality"), required=False)

    class Meta:
        model = Location
        fields = ['name', 'county', 'municipality']

    def __init__(self, *args, **kwargs):
        """
        We need access to the county field in the municipality widget, so we
        have to associate the form instance with the widget.
        """
        super(LocationForm, self).__init__(*args, **kwargs)
        self.fields['municipality'].widget.form_instance = self

And finally, the admin class:

class LocationAdmin(admin.ModelAdmin):
    form = LocationForm
    class Media:
        js = ('http://ajax.googleapis.com/ajax/libs/jquery/1.4.0/jquery.min.js',
                '/site_media/js/municipality.js')

admin.site.register(Location, LocationAdmin)

Let me know if something remains unclear.