I’m enjoying the Django framework. Sometimes, what was seems rather easy (relatively speaking) gets sort of complicated when working within the boundaries of a framework. Such was the case with my latest endeavor to filter on drop-down based on the selection of another drop-down.
Here is the situation: I have a database with car makes and models. When a user selects a make, I want to update the models drop-down with only the models associated with that make. Sounds pretty common, right? The database has a lot of entries, so I don’t want to do it with pure javascript because that could make for big script, and maybe I don’t want to give away all my data so easily so some hacker can just parse a simple javascript file to get all the make/model data out of my database (not that I care, but some people might). Therefore I want to use Ajax to populate the data.
First, the proof of concept. I created this simple html file to test how it might work:
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
<html>
   <head>
       <meta http-equiv=”Content-type” content=”text/html; charset=utf-8″ />
       <title>Update Drop-Down Without Refresh</title>
       <script type=”text/javascript” charset=”utf-8″>
           function FilterModels() {
               var makeslist = document.getElementById(’makes’);
               var modelslist = document.getElementById(’models’);
               var make_id = makeslist.options[makeslist.selectedIndex].value;
               var modelstxt = new Array();
               modelstxt[1] = “1\tEscort\n2\tTaurus”;
               modelstxt[2] = “1\tAltima\n2\tMaxima”;
               var models = modelstxt[make_id].split(”\n”);
               for (var count = modelslist.options.length-1; count >-1; count–){
                   modelslist.options[count] = null;
               }
               for (i=0; i<models.length; i++){
                   var modelvals = models[i].split(”\t”);
                   var option = new Option(modelvals[1], modelvals[2], false, false);
                   modelslist.options[modelslist.length] = option;
               }
           }
       </script>
   </head>
   <body>
       <p>This is a proof of concept to update a select (drop-down) list of values, based on the selection of another select (drop-down) element using ajax.</p>
       <form action=”" method=”get” accept-charset=”utf-8″>
           <select name=”makes” onchange=”FilterModels()” id=”makes”>
               <option>–</option>
               <option value=”1″>Ford</option>
               <option value=”2″>Nissan</option>
           </select>
           <select name=”models” id=”models”>
               <option>Choose Make</option>
           </select>
       </form>
   </body>
</html>
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That basically helped me organize the code I needed to change the drop-down, now I just need to plug in the Ajax and server side code to make it happen.
Notice, in the proof of concept, I used a tab delimited format for my models. I really didn’t want to mess with parsing xml or json or whatever. So I wrote a model feed based on my automobile make id.
URL:
(r'^feeds/models-by-make-id/(\d+)/$', 'autos.views.feed_models'),
View:
def feed_models(request, make_id):
   make = AutoMake.objects.get(pk=make_id)
   models = AutoModel.objects.filter(make=make)
   return render_to_response('feeds/models.txt', {'models':models}, mimetype="text/plain")
Template:
{% for model in models %}{{ model.id }}Â Â Â {{ model.model }}
{% endfor %}
That gave me the data feed that I needed to request via Ajax. Now for my form. Because the model field loads dynamically, I had to override the clean function in the ChoiceField class and use it instead so I didn’t get invalid choice errors:
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class DynamicChoiceField(forms.ChoiceField):Â
   def clean(self, value):Â
     return value
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class MyForm(forms.Form):Â
   make = ModelChoiceField(AutoMake.objects, widget=forms.Select(attrs={'onchange':'FilterModels();'}))Â
   model = DynamicChoiceField(widget=forms.Select(attrs={'disabled':'true'}), choices=(('-1','Select Make'),))
Notice that I am making a call to FilterModels() on my ModelChoiceField’s onchange event. So, here is that code as well:
function FilterModels(sel_val) {
var modelList = $('id_model');
for (var count = modelList.options.length-1; count >-1; count--){
modelList.options[count] = null;
}
modelList.options[0] = new Option('Loading...', '-1', false, false);
modelList.disabled = true;
var makeList = $('id_make');
var make_id = makeList.options[makeList.selectedIndex].value;
if (make_id > 0) {
new Ajax.Request('/feeds/models-by-make-id/' + make_id + '/', {
method: 'get',
onSuccess: function(transport){
var response = transport.responseText || 'no response text';
var kvpairs = response.split("\n");
for (i=0; i<kvpairs.length - 1; i++) {
m = kvpairs[i].split("\t");
var option = new Option(m[1], m[0], false, false);
modelList.options[i] = option;
}
modelList.disabled = false;
if (sel_val > 0) {
modelList.value = sel_val;
}
},
onFailure: function(){
alert('An error occured trying to filter the model list.');
modelList.options[0] = new Option('Other', '0', false, false);
modelList.disabled = false;
}
});
}
else {
modelList.options[0] = new Option('Select Make', '-1', false, false);
modelList.disabled = true;
}
}
I also make a call to this function in my template after the form loads. This is incase the form is refreshed, or it fails validation and a make is still selected. It will populate the model list and select the value that was selected previously:
<script type="text/javascript" charset="utf-8">
FilterModels({{ model_id }});
</script>
Note that in order to get the model_id, I had to set it in my view:
def add_classified(request):
if request.method == 'POST':
form = AdForm(request.POST)
if request.POST.has_key('model'):
model_id = request.POST['model']
else:
model_id = 0
return render_to_response('add_classified.html', {'form':form,'model_id':model_id}, context_instance=RequestContext(request))
I think that’s about it… Did I miss anything?
