Django Ajax现场帮助

I have a Django application where I'm trying to have a form that populates a drop down dynamically based on a previous dropdown.

simplified Models:

class VehicleMake(models.Model):
    make = models.CharField(max_length = 30)


class VehicleModel(models.Model):
    model = models.CharField(max_length = 80)
    make = models.ForeignKey(VehicleMake)

class Listing(models.Model):
    make = models.ForeignKey(VehicleMake)
    model = models.ForeignKey(VehicleModel)

Form:

class DynamicChoiceField(ModelChoiceField):
    def clean(self, value):
        return value

class MyForm(ModelForm):
    category = ModelChoiceField(VehicleCategory.objects, widget=forms.Select(attrs={'onchange':'FilterMakes();'}))
    make = DynamicChoiceField(VehicleMake.objects,widget=forms.Select(attrs={'disabled':'true','onchange':'FilterModels();'}), empty_label="Select Model")
    model = DynamicChoiceField(VehicleModel.objects,widget=forms.Select(attrs={'disabled':'true'}), empty_label="Select Make")


    class Meta:
        model = Listing

View:

def new_listing(request):
    if request.method == 'POST':
        form = MyForm(request.POST)
        if form.is_valid():
            form.save()

    else:
        form = MyForm()



    return render_to_response("newlisting.html", {
        "form": form,'model_id':model_id,'make_id':make_id
    })

I also have some ajax defined for the auto-populate but this is not the problem

When I submit the form I get the following:

Cannot assign "u'2'": "Listing.make" must be a "VehicleMake" instance.

if I try

make=VehicleMake.objects.get(pk=request.POST['make'])
form.fields['make'] = make

then I get

'VehicleMake' object has no attribute 'widget'

After the suggestion of one of the commenter's that the DynamicChoiceField class was the culprit I removed it and set the form objects for ModelChoiceFields with the exact same other parameters. The object appears to pass and validate correctly as well. The extra class existed based on an old tutorial I found. it appears that what the author did there works with the forms.ChoiceField but is not required for using a ModelChoiceField

thanks everyone for the help