Widget/field not present when custom `through` is used

[petrklus]

I need to use a custom through model with the SortedManyToManyField. However, as soon as I add the field settings, the field widget is no longer present in the admin.

My code:

class TestFeature(models.Model):
    name = models.CharField(max_length=100)

    def __str__(self):
        return self.name



class TestProductDetail(models.Model):

    name = models.CharField(max_length=100)
    features = SortedManyToManyField(
        TestFeature,
        sort_value_field_name="sort_number",
        related_name="product_details",
        through="ProductDetailFeatureThrough",
        through_fields=("testproductdetail", "testfeature"),
    )

    def __str__(self):
        return self.name


class ProductDetailFeatureThrough(models.Model):
    _sort_field_name = "sort_number"

    testfeature = models.ForeignKey(TestFeature, on_delete=models.CASCADE)
    testproductdetail = models.ForeignKey(TestProductDetail, on_delete=models.CASCADE)
    param_one = models.CharField(max_length=50, blank=True, default="")
    param_two = models.CharField(max_length=50, blank=True, default="")
    sort_number = models.IntegerField(default=0)

    def __str__(self):
        return "Relationship to {0} extra params <{1}> <{2}>".format(
            self.testfeature.name, self.param_one, self.param_two
        )

My admin is equally simple:

@admin.register(TestProductDetail)
class TestProductDetailAdmin(admin.ModelAdmin):
    pass


@admin.register(TestFeature)
class TestFeatureAdmin(admin.ModelAdmin):
    pass

When I comment out the through and through_fields, I get the widget to appear again in the admin and it works fine (but of course does not use my custom through model).

No errors are thrown in either scenario, just the field is not present.

[petrklus]

In case if anyone stumbles on this - I resolved the issue by adding the field again to Django admin manually, I could not make it work automagically without the explicit field:

from sortedm2m.forms import SortedMultipleChoiceField

class TestProductDetailAdminForm(forms.ModelForm):
    features = SortedMultipleChoiceField(
        label="Features",
        required=False,
    )
    class Meta:
        model = TestProductDetail
        fields = "__all__"

@admin.register(TestProductDetail)
class TestProductDetailAdmin(admin.ModelAdmin):
    form = TestProductDetailAdminForm