@extend_schema_field does not work on serializer used with many=True

[mraerino]

Describe the bug

I have a field serializer that has no fields (just defines to_internal_value), so any field that uses it will be missing from the schema. I'm trying to override this with @extend_schema_field but that's not working if the serializer is used with many=True.

To Reproduce

from drf_spectacular.types import OpenApiTypes
from drf_spectacular.utils import extend_schema_field
from rest_framework import serializers

@extend_schema_field(OpenApiTypes.INT64)
class RangeSerializer(serializers.Serializer):
  def to_internal_value(self, data):
    return data # usually there is more happening here

class ModelSerializer(serializers.Serializer):
  ranges = RangeSerializer(many=True)

Expected behavior

I expect the generated schema for ModelSerializer to be roughly this:

Model:
  type: object
  properties:
    ranges:
      type: array
        items:
          type: integer

---

this could be related to #694

I looked at this a little bit with a debugger and was able to find that this is related to how the many=True causes drf to wrap the serializer in a ListSerializer which hides the override annotation. In _unwrap_list_serializer (source) the code is directly calling resolve_serializer directly instead of checking for overrides again.

[tfranzel]

Hi,

unfortunately the extend_schema_field decorator is not meant to be used on serializer. As the name suggests it is meant to be used with serializer fields. extend_schema_serializer, which would be the right decorator, does not allow this kind of manipulation.

I get this is a simplified example but are you not using a IntegerField instead of RangeSerializer?

If you want this to work you need to write a serializer extension for this: https://drf-spectacular.readthedocs.io/en/latest/customization.html#declare-serializer-magic-with-openapiserializerextension

There are quite a few example usages in the codebase under contrib on how to use it.