Exception caught for serializer field ReadOnlyField with a FileField source

[jennydaman]

Describe the bug

I have a simple file server:

from django.db import models
from django.shortcuts import render
from rest_framework import generics, serializers

class FooFile(models.Model):
    fname = models.FileField(max_length=1024, unique=True)

class FooFileSerializer(serializers.HyperlinkedModelSerializer):
    fname = serializers.FileField(use_url=False, required=False)
    fsize = serializers.ReadOnlyField(source="fname.size")  # <-- herein lies the problem

    class Meta:
        model = FooFile
        fields = ("fname", "fsize")

class FooFileList(generics.ListCreateAPIView):
    http_method_names = ["get", "post"]
    serializer_class = FooFileSerializer
    queryset = FooFile.objects.all()

Generating the OpenAPI schema using drf-spectacular prints this warning:

 Warning [FooFileList > FooFileSerializer]: could not resolve field on model <class 'foo.models.FooFile'> with path "fname.size". This is likely a custom field that does some unknown magic. Maybe consider annotating the field/property? Defaulting to "string". (Exception: 'NoneType' object has no attribute '_meta')

To Reproduce

See https://github.com/jennydaman/spectacular-fsize-bug

Expected behavior

No warnings. components.schemas.FooFile.properties.fsize.type should be number.

[jennydaman]

I would like some kind of workaround, but I am not sure how one would use extend_schema_field here. For the time being, I am going to ignore the warning and use a post-processing hook to fix FooFile.properties.fsize.type.

[tfranzel]

Hi, you can do this serializers.ReadOnlyField(source="fname") but not "fname.size". We can navigate arbitrary chains of models/fields/properties, but not attributes of field.

We have a lot of navigation power in there, but even if we would have accessed size, we still could not know what type it is. It is dynamically added and a implementation detail of that specific model field.

There are a few ways to get there (e.g. typed property function on the model) with ReadOnlyField, but I would use a SerializerMethodField on the seriailzer. It is slightly more vebose, but it is clean and typed:

fsize = serializers.SerializerMethodField()

def get_fsize(self, object) -> int:
    return object.fname.size

[jennydaman]

@tfranzel Thank you for the solution!