Can I override default CharField to ChoiceField in a ModelForm?

Question

I have a (horrible) database table that will be imported from a huge spreadsheet. The data in the fields is for human consumption and is full of "special cases" so its all stored as text. Going forwards, I'd like to impose a bit of discipline on what users are allowed to put into some of the fields. It's easy enough with custom form validators in most cases.

However, there are a couple of fields for which the human interface ought to be a ChoiceField. Can I override the default form field type (CharField)? (To clarify, the model field is not and cannot be constrained by choices, because the historical data must be stored. I only want to constrain future additions to the table through the create view).

class HorribleTable( models.Model):
    ...
    foo = models.CharField( max_length=16, blank=True, ... )
    ... 

class AddHorribleTableEntryForm( models.Model)
     class Meta:
          model = HorribleTable
          fields = '__all__' # or a list if it helps
      
     FOO_CHOICES = (('square', 'Square'), ('rect', 'Rectangular'), ('circle', 'Circular') )

     ...?

Answer 1

Perhaps you could render the forms manually, passing the options through the context and make the fields in html.

Take a look at here:https://docs.djangoproject.com/en/4.0/topics/forms/#rendering-fields-manually

Answer 2

I think you can easily set your custom form field as long it will match the data type with the one set in your model (e.g. do not set choices longer than max_length of CharField etc.). Do the following where foo is the same name of the field in your model:

class AddHorribleTableEntryForm(forms.ModelForm):
    foo  = forms.ChoiceField(choices=FOO_CHOICES)

    class Meta:
        model = HorribleTable
        ...

I think this is perfectly fine for a creation form. It's will not work for updates as the values in the DB will most probably not match your choices. For that, I suggest adding a second form handling data updates (maybe with custom permission to restrict it).

UPDATE

Another approach will be to override the forms init method. That way you can handle both actions (create and update) within the same form. Let the user select from a choice field when creating an object. And display as a normal model field for existing objects:

class AddHorribleTableEntryForm(forms.ModelForm):
    foo  = forms.ChoiceField(choices=FOO_CHOICES)

    class Meta:
        model = HorribleTable
        fields = '__all__' # or a list if it helps

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        instance = kwargs.get("instance", None)
        if instance is None:
            self.fields["foo"].widget = forms.widgets.Select(choices=self.FOO_CHOICES)