How can snippet forms be customized?

Question

This part of the wagtail documentation shows how to customize the form of Page models by subclassing WagtailAdminPageForm and pass the subclass as base_form_class to the Page model.

It also says:

Custom forms for snippets must subclass WagtailAdminModelForm, and custom forms for pages must subclass WagtailAdminPageForm.

However when I try the same using a snippet I get the following error:

ModelForm has no model class specified.

My code:

class ScheduleAdminForm(WagtailAdminModelForm):
    def __init__(self, *args, **kwargs):
        print("Constructor of ScheduleAdminForm")
        super().__init__(*args, **kwargs)


class Schedule(ClusterableModel):
    name = models.CharField(max_length=32)
    base_form_class = ScheduleAdminForm

I also tried to add the model in the Meta class of the ScheduleAdminForm. However then I have a circular use and I can't use the model name in quotes for Meta.model as can be done for model fields.

Answer 1

You can set the model's base_form_class attribute outside the main class definition:

class Schedule(ClusterableModel):
    name = models.CharField(max_length=32)


class ScheduleAdminForm(WagtailAdminModelForm):
    def __init__(self, *args, **kwargs):
        print("Constructor of ScheduleAdminForm")
        super().__init__(*args, **kwargs)

    class Meta:
        model = Schedule
        fields = ["name"]


Schedule.base_form_class = ScheduleAdminForm

Answer 2

It is definitely possible to declare as you've done, I do this myself:

    from core.forms.restricted_panels_admin_forms import \
        RestrictedPanelsAdminModelForm
    
    @register_snippet
    class StoreDepartment(ClusterableModel):
        base_form_class = RestrictedPanelsAdminModelForm
        ....

RestrictedPanelsAdminModelForm subclasses WagtailAdminModelForm

Only difference I can see is that it's importing the form, can't imagine that's the cause though.