Django model choice field - depend on other field's choice

You probably can't because it depends of user interaction with your form: your server can't know in advance which element your user will select before sending the form to the browser. You could probably achieve this using ajax. I think a working process could be :

• Create a form with all the fields, and make make field hidden
• Create a view (I'll call it AjaxMakeFieldView) that will catch an ajax request taking a vehicle_type argument and return the HTML for make field, populated with relevant data. Add a URL in your URLConf for this view.
• In your template, add a Javascript binding : when user select a vehicle_type, the browser will send aan ajax request to AjaxMakeFieldView and replace hidden make field with returned HTML

If you don't want javascript, another way would be a two step form :

• A first form with a vehicle_type field
• Once the first form is submitted, your user get a second form with a make field, which initial data is populated depending of vehicle_type selected in the first form.

I've never done this, but Django documentation on Form wizard seems a good place to start.