How to use ArrayField with a custom field as a child?

[rijenkii]

I have a model with a field = ArrayField(CustomField()).
I have replaced the CustomField with a copy of UUIDField from Djangos source code for this example.

models.py

import uuid

from django.contrib.postgres.fields import ArrayField
from django.core import exceptions
from django.db import models
from django.utils.translation import gettext_lazy as _


class CustomUUIDField(models.Field):
    default_error_messages = {
        "invalid": _("“%(value)s” is not a valid UUID."),
    }
    description = _("Universally unique identifier")
    empty_strings_allowed = False

    def __init__(self, verbose_name=None, **kwargs):
        kwargs["max_length"] = 32
        super().__init__(verbose_name, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        del kwargs["max_length"]
        return name, path, args, kwargs

    def get_internal_type(self):
        return "UUIDField"

    def get_prep_value(self, value):
        value = super().get_prep_value(value)
        return self.to_python(value)

    def get_db_prep_value(self, value, connection, prepared=False):
        if value is None:
            return None
        if not isinstance(value, uuid.UUID):
            value = self.to_python(value)

        if connection.features.has_native_uuid_field:
            return value
        return value.hex

    def to_python(self, value):
        if value is not None and not isinstance(value, uuid.UUID):
            input_form = "int" if isinstance(value, int) else "hex"
            try:
                return uuid.UUID(**{input_form: value})
            except (AttributeError, ValueError):
                raise exceptions.ValidationError(
                    self.error_messages["invalid"],
                    code="invalid",
                    params={"value": value},
                )
        return value


class Test(models.Model):
    field = ArrayField(CustomUUIDField(), blank=True)

urls.py

from django.urls import include
from django.urls import path
from rest_framework import routers
from rest_framework import serializers
from rest_framework import viewsets

from .models import Test


class TestSerializer(serializers.ModelSerializer):
    class Meta:
        model = Test
        fields = ("field",)


class TestViewSet(viewsets.ModelViewSet):
    queryset = Test.objects.all()
    serializer_class = TestSerializer


router = routers.DefaultRouter()
router.register(r"test", TestViewSet)

urlpatterns = [
    path("", include(router.urls)),
]

When adding a new Test object, or retrieving an existing one, this error is thrown:

Exception

Traceback (most recent call last):
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/viewsets.py", line 125, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/views.py", line 509, in dispatch
    response = self.handle_exception(exc)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/mixins.py", line 20, in create
    headers = self.get_success_headers(serializer.data)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/serializers.py", line 555, in data
    ret = super().data
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/serializers.py", line 253, in data
    self._data = self.to_representation(self.instance)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/serializers.py", line 522, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/fields.py", line 1626, in to_representation
    return [self.child.to_representation(item) if item is not None else None for item in data]
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/fields.py", line 1626, in <listcomp>
    return [self.child.to_representation(item) if item is not None else None for item in data]
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/rest_framework/fields.py", line 1875, in to_representation
    value = self.model_field.value_from_object(obj)
  File "/home/rijenkii/.pyenv/versions/3.11.0/lib/python3.11/site-packages/django/db/models/fields/__init__.py", line 1059, in value_from_object
    return getattr(obj, self.attname)

Exception Type: AttributeError at /test/
Exception Value: 'UUID' object has no attribute 'field'

What am I doing wrong?

---

EDIT:

Exception disappears when I add this to the serializer:

field = serializers.ListField(child=serializers.CharField(max_length=32), allow_empty=True)

But it doesn't feel right, and does not throw 400 validation errors anymore -- just straight up 500s.

Just in case, I am using that CustomField in the project already with no problems.

---

EDIT 2:

Oh, and just in case again, versions:

>>> import django
>>> import rest_framework
>>> django.__version__
'4.1.3'
>>> rest_framework.__version__
'3.14.0'

[tomchristie]

Sounds like you need to be using a serializer field, not a Django model field.

https://www.django-rest-framework.org/api-guide/fields/

[rijenkii]

But the custom field works when used without ArrayField, with validation errors and all.
I would have expected it to work here as well.