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    <p>With a little hackery, it's quite doable.</p> <p>In the following example, <strong>County</strong> is used instead of <strong>State</strong> and <strong>Municipality</strong> instead of <strong>City</strong>. So, the models are as follows:</p> <pre><code>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")) </code></pre> <p>There are two sides of the problem: client-side JavaScript and server side field rendering.</p> <p>The client side JavaScript (with JQuery, assumed to be served from <strong>/site_media/js/municipality.js</strong>) is as follows:</p> <pre><code>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 = '&lt;option value="" selected="selected"&gt;---------&lt;/option&gt;'; for (var i in ret) { options += '&lt;option value="' + ret[i].id + '"&gt;' + ret[i].name + '&lt;/option&gt;'; } response_cache[county_id] = options; $("#id_municipality").html(options); }); } } $(document).ready(function() { $("#id_county").change(function() { fill_municipalities($(this).val()); }); }); </code></pre> <p>Now you need the Ajax view for serving municipalities that belong to a given county (assumed to be served from <strong>/municipalities_for_county/</strong>):</p> <pre><code>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'}) </code></pre> <p>And finally the server side code in <em>admin.py</em> for rendering the field is as follows. First, the imports:</p> <pre><code>from django import forms from django.forms import widgets from django.forms.util import flatatt from django.utils.encoding import smart_unicode from django.utils.safestring import mark_safe from django.contrib import admin from django.utils.translation import ugettext_lazy from myproject.places.models import Municipality, Location </code></pre> <p>Then, the widget:</p> <pre><code>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: for m in model_obj.county.municipality_set.all(): self.choices.append((m.id, smart_unicode(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_unicode(m))) # copy-paste from widgets.Select.render final_attrs = self.build_attrs(attrs, name=name) output = [u'&lt;select%s&gt;' % flatatt(final_attrs)] options = self.render_options(choices, [value]) if options: output.append(options) output.append('&lt;/select&gt;') return mark_safe(u'\n'.join(output)) </code></pre> <p>Next, the form:</p> <pre><code>class LocationForm(forms.ModelForm): municipality = forms.ModelChoiceField(Municipality.objects, widget=MunicipalityChoiceWidget(), label=ugettext_lazy("Municipality"), required=False) class Meta: model = Location 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 </code></pre> <p>And finally, the admin class:</p> <pre><code>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) </code></pre> <p>Let me know if something remains unclear.</p>
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    1. CO+1 for such an awesomely thorough reply. I'm going to do something similar and although I'm not using the admin, this should be of great help. Thanks!
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    2. COthank you - a gentleman and scholar.
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