how to overide in forms queryset none() attribute and somehow allow to save the field?
class Visit(Model):
    reference_visit = models.ForeignKey('self',
        help_text="Visit needs a refrence to Prior Visits",
        null=True, blank=True)
    show_prior_responses = models.BooleanField(default=False,
        help_text="Show PriorResponses")

# has many field but i am making it short.
def __unicode__(self):
    result = """Visit id:%s pt:%s""" % (self.id, self.patient.id)
    return result

class VisitSetupForm(Form):
    list_visit_ids = ModelChoiceField(
        queryset=Visit.objects.none(),
        empty_label='Select Revisit ID',required=False)
    show_prior_visit = ModelChoiceField(
        queryset=User.objects.all(),
        empty_label="Select User for Revisit",required = False)

def setup(request):
    """
    Allow an Admin user the ability to setup a patient & visit all at once.
    """
    if request.user.is_superuser:
        form_class = AdminVisitSetupForm
        all_topics = True
    else:
        form_class = VisitSetupForm
        all_topics = False

    f = form_class()

    # Get a list of topics for each report.
    report_topics = {}

    for r in Interview.objects.all():
        report_topics[r.id] = [t['ad'] for t in r.topics.values('ad')]


    data = {
        'superuser':request.user.is_superuser,
        'report_topics':simplejson.dumps(report_topics)
    }

    try:
        request.user.reviewer
        data['reviewer'] = True
    except:
        pass

    if request.method == "POST":
        f = form_class(request.POST)

        if f.is_valid():
            # Create the patient, generate a password, and send them on their way.
            cd = f.cleaned_data

            patient = None
            if cd['revisit']:
                # Check for an existing user first.
                try:
                    patient = Patient.objects.get(username=cd['username'])
                except Patient.DoesNotExist, e:
                    data['form'] = f
                    data['msg'] = 'There is no user with this username.'
                    return render_to_response('visit/setup.html', data, context_instance=RequestContext(request))

            admin_user = get_user(request)
            organization = None
            if admin_user:
                organization = admin_user.organization

            if patient and not request.user.is_superuser:
                # Make sure the patient they've selected is one of their own.
                if patient.organization != organization:
                    return HttpResponseForbidden('You are not allowed to see this page.')

            if not patient:
                password = generate_password()
                user = User.objects.create_user(cd['username'], cd['contact_email'], password)
                user.first_name = cd['first_name']
                user.last_name = cd['last_name']
                user.save()
                patient = Patient(
                    user=user,
                    username=user.username,
                    contact_phone=cd['contact_phone'],
                    date_of_birth=cd['date_of_birth'],
                    email=user.email,
                    first_name=user.first_name,
                    gender=cd['gender'],
                    last_name=user.last_name,
                    maiden_name=cd['maiden_name'],
                    organization=organization,
                    patient_type=cd['patient_type'],
                    security_answer=cd['security_answer'],
                    security_question=cd['security_question'],

                )
                patient.save()

                # Send them an email.
                t = loader.get_template('www/new_account.txt')
                c = Context({
                    'password':'%s-%s-%s' % (password[:3], password[3:5], password[5:]),
                    'patient':patient
                })
                msg = t.render(c)

                try:
                    send_mail(
                        'A request by your physician to do an online medical history before your appointment.',
                        msg,
                        'support@careprep.com',
                        [user.email]
                    )
                except Exception, e:
                    log.error('Could not send email for new account %s because: [%s]' % (user.username, e))

                request.session['password'] = password

            # Create the Visit, too.
            interview = cd['interview']

            list_visit_ids = cd['list_visit_ids']
            print list_visit_ids

            visit = Visit(
                    reference_visit = cd['list_visit_ids'],
                    show_prior_responses = cd['show_prior_responses'],
                    patient=patient
                )

            if request.user.is_superuser:
                topics = cd['topics']
            else:
                topics = set(list(interview.topics.all()) + list(cd['topics']))

            reviewer_mode = cd.get('reviewer_mode') or patient.patient_type == 'Reviewer'

            url, visit = initialize_visit(
                                request,
                                patient=patient,
                                starting_section=interview.starting_section,
                                visit_title='%s %s' % (patient, interview.title),
                                topics=topics,
                                reviewer_mode=reviewer_mode,
                                chief_complaint=cd['chief_complaint'],
                                location=cd['interview_site'],
                                reference_visit = cd['list_visit_ids'],
                                show_prior_responses  = cd['show_prior_responses'],
                              )

            next_url = "/visit/confirmation/%s/%s/?next=%s" % (patient.user.id, interview.id, url)
else:
     v = Visit.objects.get(pk=request.POST['list_visit_ids'])
                print v

            return HttpResponseRedirect(next_url)
# all the fields that are not given pls ignore.

class MyForm(forms.Form):
    my_input = forms.CharField()

    def clean_my_input(self):
        input = self.cleaned_data.get('my_input')
        try:
             return  MyModel.objects.get(pk=input) # add a filter here if you want
             # (whatever filters you were using in the queryset argument)
        except MyModel.DoesNotExist:
             raise forms.ValidationError("Doesn't exist / is invalid")
        return input