Today I ran into an issue where I wanted to change the options for a form field based upon what was selected in other fields. Employers use this particular form to create a job opening. First, they select the type of job they're creating (e.g. Waitstaff, Flight Attendant, Programmer, etc.) and then they choose from a list of hiring policies that depends on the type of job.

By default Django's ModelForm sees the hiring policy foreign key for the job opening and lets you select from all of hiring policies for any type of job. I found out how to filter form querysets from StackOverflow. The next issue was how to actually do the updating once a user chose a job type.

I ended up adding some Javascript to POST when the job type selection changed. This worked exactly as needed. The only issue is that because the form is (usually) incompletely filled out, the page was covered in "field required" errors. Someone in the #django IRC channel suggested hiding the errors on the page, but I couldn't find any good way of suppressing these.

So, Form(request.POST) leads to errors all over the page.

My next try was with the form's initial argument. This is one way to give the form data without actually processing it. The thing is, Form(initial=request.POST) doesn't do what you think it does. If you do that, all your form fields will end up filled with Python list literals. request.POST is a QueryDict and not a normal dict. Its contents look something like:

{u'salary': [u''], u'city': [u''], u'zip': [u''], ... }

Because queries can post multiple values for the same key, the values in the QueryDict are lists of values. I just want a normal dict to pass to initial though. The solution?

Form(initial=dict(request.POST.items()))

Why?

>>> request.POST
<QueryDict: {u'salary': [u'500', u'600'], u'city': [u''], ... u'salary_type': [u'']}>
>>> dict(request.POST.items())
{u'salary': u'600', u'city': u'', ... u'salary_type': u''}

http://stackoverflow.com/questions/291945/how-do-i-filter-foreignkey-choices-in-a-django-modelform