Populating CheckboxSelectMultiple widget using my own model in Wagtail admin

Question

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:

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.

Populating CheckboxSelectMultiple widget using my own model in Wagtail admin

Context

Problem

Solution attempts

Similar questions that didn't address my problem

Answers (1)

Field Type

Model

Model Form

Overriding the context

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