Make a field required if another field is checked in Django form

Question

I have a model like this:

class News(models.Model):
    is_activity = models.BooleanField(default=False)
    activity_name = models.CharField(max_length=240, blank=True, null=True)

What I am trying to achieve is, if is_activity is checked in I want activity_name to be required. Thus, I am trying to override the __init__ method:

class NewsForm(forms.ModelForm):
    class Meta:
        model = News

    def __init__(self, *args, **kwargs):
        super(NewsForm, self).__init__(*args, **kwargs)
        if self.fields['is_activity'] is True:
            self.fields['activity_name'].required = True


class NewsAdmin(FrontendEditableAdminMixin, admin.ModelAdmin):
    form = NewsForm

Even if I check in the is_activity the activity_name is non-required. What's wrong?

Answer 1

The ModelForm.clean() method gives you access to the cleaned data – this is where you can include the field-specific conditional logic:

from django.core.validators import EMPTY_VALUES

class NewsForm(forms.ModelForm):
    class Meta:
        model = News

    def clean(self):
        is_activity = self.cleaned_data.get('is_activity', False)
        if is_activity:
            # validate the activity name
            activity_name = self.cleaned_data.get('activity_name', None)
            if activity_name in EMPTY_VALUES:
                self._errors['activity_name'] = self.error_class([
                    'Activity message required here'])
        return self.cleaned_data

class NewsAdmin(FrontendEditableAdminMixin, admin.ModelAdmin):
    form = NewsForm