Cannot validate dynamic choices with Django ModelForm

Question

I have a Django ModelForm in Google App Engine with a ChoiceField, let's say location:

class MyForm(ModelForm):
    location = ChoiceField(label="Location")

    class Meta:
        model = MyModel

In order to dynamically add the choices for location, and not have issues with app caching, I add them after the form has initialized:

form = MyForm(request.POST, instance=my_instance)
form.fields['location'].choices = Location.all().fetch(1000)

The problem I'm having now is that when the form is initialized via the data in request.POST the choices do not yet exist and I am receiving an error stating that an invalid choice is made (since the value does not yet exist in the list of choices).

I don't like that validation is occurring when I am initializing the form instead of waiting until I call form.is_valid(). Is there any way to suppress validation during my object instantiation? Or some other way to fix this?

UPDATE: I'm pretty sure ModelFormMetaclass is causing me my grief by validating the provided instance when the form is created. Still not sure how to fix though.

Thanks!

Answer 1

There must be other ways to do this, but possibly the most straightforward is to add the field in the form's __init__() method:

class MyForm(ModelForm):
    ...
    def __init__(self, *args, **kwargs):
        try:
            dynamic_choices = kwargs.pop('dynamic_choices')
        except KeyError:
            dynamic_choices = None # if normal form
        super(MyForm, self).__init__(*args, **kwargs)
        if dynamic_choices is not None:
            self.fields['location'] = ModelChoiceField(
                                          queryset=dynamic_choices)
    class Meta:
        model = MyModel

And your view would look something like:

def my_view(request):
    locations = Location.objects.all() # or filter(...) or whatever
    dynamic_form = MyForm(dynamic_choices=locations)

    return direct_to_template(request, 
                              'some_page.html',
                              {'form': dynamic_form},)

Let us know how that works for you.