Django/jQuery Cascading Select Boxes?

Here is my solution. It uses the undocumented Form method _raw_value() to peek into the data of the request. This works for forms, which have a prefix, too.

class CascadeForm(forms.Form):
    parent=forms.ModelChoiceField(Parent.objects.all())
    child=forms.ModelChoiceField(Child.objects.none())

    def __init__(self, *args, **kwargs):
        forms.Form.__init__(self, *args, **kwargs)
        parents=Parent.objects.all()
        if len(parents)==1:
            self.fields['parent'].initial=parents[0].pk

        parent_id=self.fields['parent'].initial or self.initial.get('parent') \
                  or self._raw_value('parent')
        if parent_id:
            # parent is known. Now I can display the matching children.
            children=Child.objects.filter(parent__id=parent_id)
            self.fields['children'].queryset=children
            if len(children)==1:
                self.fields['children'].initial=children[0].pk

jquery Code:

function json_to_select(url, select_selector) {
/*
 Fill a select input field with data from a getJSON call
 Inspired by: http://stackoverflow.com/questions/1388302/create-option-on-the-fly-with-jquery
*/
    $.getJSON(url, function(data) {
    var opt=$(select_selector);
    var old_val=opt.val();
        opt.html('');
        $.each(data, function () {
            opt.append($('<option/>').val(this.id).text(this.value));
        });
        opt.val(old_val);
        opt.change();
    })
}


   $(function(){
     $('#id_parent').change(function(){
       json_to_select('PATH_TO/parent-to-children/?parent=' + $(this).val(), '#id_child');
     })  
    });

Callback Code, which returns JSON:

def parent_to_children(request):
    parent=request.GET.get('parent')
    ret=[]
    if parent:
        for children in Child.objects.filter(parent__id=parent):
            ret.append(dict(id=child.id, value=unicode(child)))
    if len(ret)!=1:
        ret.insert(0, dict(id='', value='---'))
    return django.http.HttpResponse(simplejson.dumps(ret), 
              content_type='application/json')

You could set a hidden field to have the real "state" value, then use jQuery to create the <select> list and, on .select(), copy its value to the hidden field. Then, on page load, your jQuery code can fetch the hidden field's value and use it to select the right item in the <select> element after it's populated.

The key concept here is that the State popup menu is a fiction created entirely in jQuery and not part of the Django form. This gives you full control over it, while letting all the other fields work normally.

EDIT: There's another way to do it, but it doesn't use Django's form classes.

In the view:

context = {'state': None, 'countries': Country.objects.all().order_by('name')}
if 'country' in request.POST:
    context['country'] = request.POST['country']
    context['states'] = State.objects.filter(
        country=context['country']).order_by('name')
    if 'state' in request.POST:
        context['state'] = request.POST['state']
else:
    context['states'] = []
    context['country'] = None
# ...Set the rest of the Context here...
return render_to_response("addressform.html", context)

Then in the template:

<select name="country" id="select_country">
    {% for c in countries %}
    <option value="{{ c.val }}"{% ifequal c.val country %} selected="selected"{% endifequal %}>{{ c.name }}</option>
    {% endfor %}
</select>

<select name="state" id="select_state">
    {% for s in states %}
    <option value="{{ s.val }}"{% ifequal s.val state %} selected="selected"{% endifequal %}>{{ s.name }}</option>
    {% endfor %}
</select>

You'll also need the usual JavaScript for reloading the states selector when the country is changed.

I haven't tested this, so there are probably a couple holes in it, but it should get the idea across.

So your choices are:

Use a hidden field in the Django form for the real value and have the select menus created client-side via AJAX, or
Ditch Django's Form stuff and initialize the menus yourself.
Create a custom Django form widget, which I haven't done and thus will not comment on. I have no idea if this is doable, but it looks like you'll need a couple Selects in a MultiWidget, the latter being undocumented in the regular docs, so you'll have to read the source.

Django/jQuery Cascading Select Boxes?

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