Model @property and SlugRelatedField not handled properly

[sparktx-adam-gleason]

In v0.24.2 this works properly but v0.25.0 and onward broke how SlugRelatedField and @properties are being handled.

Describe the bug
SlugRelatedField that reference a model @property are not being handled.

While stepping through the functions on how this field is being resolved, I tracked down how this field is being handled in v0.25.0.

First, this logic was updated in v0.25.0 compared to v0.24.2.
https://github.com/tfranzel/drf-spectacular/blob/master/drf_spectacular/openapi.py#L663

The SlugRelatedField will get to this function.

drf-spectacular/drf_spectacular/plumbing.py

Line 500 in 7392ba9

def _follow_field_source(model, path: List[str]):

and will finally return this value at the end of traversal.

drf-spectacular/drf_spectacular/plumbing.py

Lines 508 to 509 in 7392ba9

	if isinstance(field_or_property, property):
	return field_or_property.fget

Ultimately, this assertion is failing. https://github.com/tfranzel/drf-spectacular/blob/0.25.0/drf_spectacular/openapi.py#L544

Prior to this logic update, SlugRelatedFields used this logic to resolve. https://github.com/tfranzel/drf-spectacular/blob/0.24.2/drf_spectacular/openapi.py#L698-L699

To Reproduce

class MyModel(models.Model):
    label = models.CharField(
        max_length=100,
        unique=True,
    )

    class Meta:
        verbose_name = _("MyModel")
        verbose_name_plural = _("MyModels")

    def __str__(self):
        return self.label

    @property
    def property_field(self):
        return "42"

# How the field is defined in a Serializer
forty_two = serializers.SlugRelatedField(
    slug_field="property_field",
    read_only=True,
)

API Response

{
    "forty_two": "42"
}

Expected behavior
SlugRelatedFields that use @properties don't fail assertion and are resolved the same way they are handled in v0.24.2

[tfranzel]

I see, will look into it. The change happened here #854.

Before, we had it hard-coded as string, so there was little chance of it breaking. Now, the introspection is assuming a model field, but it looks like a model property is also viable. We have the mechanics for that, but somehow it is not triggered.

I'm not exactly sure why you chose a slug here, but we should support it if it works. In the meantime, this should work too (if you add a return type property_field(self)-> str:):

forty_two = serializers.ReadOnlyField(source='property_field')

[sparktx-adam-gleason]

@tfranzel For my use case, I am using SlugRelatedField because the actual property value is sourced from a ForeignKey related field to my model which I'm building a ModelSerializer for.

my_model.related_model.property_field

If I point the SlugRelatedField to a non-property field on the related model, I don't see an error in schema generation. This is to be expected because the assertion passes.

[tfranzel]

For my use case, I am using SlugRelatedField because the actual property value is sourced from a ForeignKey related

Yes, I get that. ReadOnlyField also supports relations. What I meant was serializers.ReadOnlyField(source='related_model.property_field')

If I point the SlugRelatedField to a non-property field on the related model, I don't see an error in schema generation.

Sure, because the mechanics falsely expect a model field for SlugRelatedField target.

Anyways, this fix should do the trick. Please test it if you don't mind.

P.S. Please not note that this also requires a type hint on property_field to work properly.

[tfranzel]

closed with release 0.26.0