Populating CheckboxSelectMultiple widget using my own model in Wagtail admin

Context

I've created a model, corresponding field model, and intend to reuse the built-in CheckboxSelectMultiple widget for use within the Wagtail admin. The concept is a multiple-select permission field that is saved as a bit-field:

# Model class
class Perm(IntFlag):
    Empty  = 0
    Read   = 1
    Write  = 2

I used Django's model field's documentation to create a field model that can translate my Perm type to and from my database (saved as an integer field that bitwise OR's the respective permission bits):

# Model field class
class PermField(models.Field):
    description = "Permission field"
    def __init__(self, value=Perm.Empty.value, *args, **kwargs):
        self.value = value
        kwargs["default"] = Perm.Empty.value
        super().__init__(*args, **kwargs)
    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        args += [self.value]
        return name, path, args, kwargs
    def db_type(self, connection):
        return "bigint" # PostgresSQL
    def from_db_value(self, value, expression, connection):
        if value is None:
            return Perm.Empty
        return Perm(value)
    def to_python(self, value):
        if isinstance(value, Perm):
            return value
        if isinstance(value, str):
            return self.parse(value)
        if value is None:
            return value
        return Perm(value)
    def parse(self, value):
        v = Perm.Empty
        if not isinstance(ast.literal_eval(value), list):
            raise ValueError("%s cannot be converted to %s", value, type(Perm))
        for n in ast.literal_eval(value):
            v = v | Perm(int(n))
        return v

Then, I also created a Wagtail snippet to use this new field and type:

perm_choices = [
    (Perm.Read.value, Perm.Read.name),
    (Perm.Write.value, Perm.Write.name)
]

@register_snippet
class Permission(models.Model):
    name = models.CharField(max_length=32, default="None")
    perm = PermField()
    panels = [FieldPanel("perm", widget=forms.CheckboxSelectMultiple(choices=perm_choices))]

Problem

Creating new snippets works fine, but editing an existing one simply shows an empty CheckboxSelectMultiple widget:

Empty Wagtail CheckboxSelectMultiple upon editing an existing snippet

Solution attempts

I clearly need to populate the form when it's initialised. Ideally, making use of the built-in CheckboxSelectMultiple widget. To do that, I tried defining the following form:


@register_snippet
class Permission(models.Model):
    # ...

# Custom form subclass for snippets per documentation
# https://docs.wagtail.org/en/v2.15/advanced_topics/customisation/page_editing_interface.html
class Permission(WagtailAdminModelForm):
    p = forms.IntegerField(
        widget=forms.CheckboxSelectMultiple(
            choices=perm_choices,
        ),
        label="Permission field",
    )
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['p'].initial = {
            k.name for k in [Perm.Read, Perm.Write] if k.value in Perm(int(p))
        }

    def clean_selected_permissions(self):
        selected_p = self.cleaned_data["p"]
        value = Perm.Empty
        for k in selected_p:
            value |= Perm.__members__[k]
        return value

    class Meta:
        model=Permission
        fields=["perm"]

# Models not defined yet error here!
Permission.base_form_class = PermissionForm

However, I cannot get this form to work. There's a cycle where PermissionForm requires Permission to be defined or vice-versa. Using a global model form assignment as seen here by gasman did not work for me. I'm also wondering if there's a simpler approach to solving the problem I'm facing that I'm just not seeing.

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