[Python] 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:



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.

Similar questions that didn't address my problem

• Question: Populate CheckboxSelectMultiple with existing data from django model form
  • Comment: OP implements a custom ModelForm, which links the CheckBoxSelectMultiple right up to a models.ManyToManyField. This works since the ManyToManyField type is automatically compatible with the widget. In my case, I have to set it up myself.
• Question: Initial values for CheckboxSelectMultiple
  • Comment: OP is using a MultiSubscriptionForm, which itself contains a keyword argument for populating the existing fields. This does not exist in my situation.
• Question: Django Admin Template Overriding: Displaying checkboxselectmultiple widget
  • Comment: OP describes a table structure and asks if it is possible. No answer provided solves the problem (arguably poorly phrased question)
• Question: Django - Render CheckboxSelectMultiple() widget individually in template (manually)
  • Comment: OP wants to customise the CheckboxSelectMultiple template in order to show a special arrangement. The answers provide template HTML to do this, but otherwise rely on the ManyToMany field type/relation that automagically links/fills the checkboxes.

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