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.