Django: Saving Foreign Key from Form

This is a continuation of this question in which I am trying to figure out how to construct a PointField composed of lat / lon FloatFields. I have taken @Simon's advice and restructured my model to look like this:

class Point(models.Model):
   lat = models.FloatField()
   lon = models.FloatField()

class Thing(models.Model):
   point = models.ForeignKey(Point)

My form has two fields corresponding to values from google maps' longitude and latitude coordinates:

class StepThreeForm(forms.Form):
    lat = forms.FloatField()
    lon = forms.FloatField()
    ...

However, this does not work for obvious reasons, but I am not sure how to fix it. To clarify, I am trying to have two form fields corresponding to the foreign key values of lat and lon. Here is the supplementary information (I am using a FormWizard and forms.Form):

url(r'^mapform/$', login_required(MyWizard.as_view([StepOneForm, StepTwoForm, StepThreeForm])), name='create'),

class MyWizard(SessionWizardView):  ## this view also serves to edit existing objects and provide their instances
    def done(self, form_list, **kwargs):
        id = form_list[0].cleaned_data['id']
        try:
            thing = Thing.objects.get(pk=id)
            instance = thing
        except:
            thing = None
            instance = None
        if thing and thing.user != self.request.user:
            raise HttpResponseForbidden()
        if not thing:
            instance = Thing()
            for form in form_list:
                for field, value in form.cleaned_data.iteritems():
                    setattr(instance, field, value)
            instance.user = self.request.user
            instance.save()
        return render_to_response('wizard-done.html', {
                'form_data': [form.cleaned_data for form in form_list],})

I appreciate any and all advice and help!

EDIT: Update based on Yuji Tomita's input. Most of it made a lot of sense (Thank you!), but I'm not sure why it results in a ValueError.

class MyWizard(SessionWizardView):
     ....
            for form in form_list:
                form.save(instance)
     ...

class StepOneForm(forms.Form):
     ...
     def save(self, thing):
        for field, value in self.cleaned_data.items():
            setattr(thing, field, value)

class StepTwoForm(forms.Form):
     ...
     def save(self, thing):
        for field, value in self.cleaned_data.items():
            setattr(thing, field, value)

I believe that I should keep the form fields as lat and lon, because I am using a google map in my form and taking the lat and lon from a selected input, then constructing a point fields from those values:

class StepThreeForm(forms.Form):
    lat = forms.FloatField()
    lon = forms.FloatField()

    def save(self, thing):
        thing.point = Point.objects.get_or_create(lat=self.cleaned_data.get('lat'), lon=self.cleaned_data.get('lon'))

This yields a ValueError: Cannot assign "(<Point: Point object>, False)": "Thing.point" must be a "Point" instance.

Traceback:
File "/lib/python2.7/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/lib/python2.7/django/contrib/auth/decorators.py" in _wrapped_view
  20.                 return view_func(request, *args, **kwargs)
File "/lib/python2.7/django/views/generic/base.py" in view
  48.             return self.dispatch(request, *args, **kwargs)
File "/lib/python2.7/django/contrib/formtools/wizard/views.py" in dispatch
  223.         response = super(WizardView, self).dispatch(request, *args, **kwargs)
File "/lib/python2.7/django/views/generic/base.py" in dispatch
  69.         return handler(request, *args, **kwargs)
File "/lib/python2.7/django/contrib/formtools/wizard/views.py" in post
  286.                 return self.render_done(form, **kwargs)
File "/lib/python2.7/django/contrib/formtools/wizard/views.py" in render_done
  328.         done_response = self.done(final_form_list, **kwargs)
File "/myproject/myapp/forms.py" in done
  93.       form.save(instance)
File "/myproject/myapp/forms.py" in save
  67.         thing.point = Thing.objects.get_or_create(lat=self.cleaned_data.get('lat'), lon=self.cleaned_data.get('lon'))
File "/lib/python2.7/django/db/models/fields/related.py" in __set__
  366.                                  self.field.name, self.field.rel.to._meta.object_name))

I'd recommend building a save method on each of your forms which knows how to save itself to the database. It follows a general pattern that "the form performs its action via form.save()" so it should be intuitive to follow.

The bottom line is that right now you have a blanket: "for every field in all forms, set the Thing attribute to those fields".

Since in reality you have per-form save behavior, I think it makes sense to require passing the instance to each form so that each form has a chance to save data in the way appropriate for its fields.

class Form1(...):
   def save(self, thing):
      for field, value in self.cleaned_data.items():
          setattr(thing, field, value)

class Form2(...):
   def save(self, thing):
      thing.point = Point.objects.get_or_create(lat=self.cleaned_data.get('lat'), long=...)
      # note, you may not want get_or_create if you don't want to share points.

Your view would then become:

for form in form_list:
    form.save(instance)

Just an idea.

If you want to be more DRY about it and like the automation of your other forms, I'd build a base form which has a save method already defined:

class BaseSaveBehaviorForm(forms.Form):
     def save(self, thing):
         for field, value in self.cleaned_data.items():
             setattr(thing, field, value)

class NormalBehaviorForm(BaseSaveBehaviorForm):
     # your forms as usual


class SpecialSaveBehaviorForm(forms.Form):
     def save(self, instance):
         # do something unusual