[server] schema/doc/serializers: replace serializers.MethodField with Serializer class

[gnuletik]

Currently, MethodSerializers are used to return dicts. For example, in audio_url and waveform. Other occurrences may exist.

The issue with this technique is that the auto-generated schema do not have knowledge of the return type of the method. So, in the schema, MethodFields have a string type. e.g waveform is a string https://sandbox.wasabi.telemeta.org/timeside/redoc/#operation/RetrieveItem

In order to workaround this, we should use nested serializers. In order to keep the initial implementation, the .to_representation of the serializer method can be overridden.

[Tointoin]

How to pass serializer's model item as argument such as obj argument of SerializerMethodField would do in a HyperlinkedModelSerializer ?

class WaveformSerializer(serializers.Serializer):
    """Populate waveform"""
    ... # Fields definition
    d̶e̶f̶ ̶_̶_̶i̶n̶i̶t̶_̶_̶(̶s̶e̶l̶f̶,̶ ̶i̶t̶e̶m̶)̶:̶
        s̶e̶l̶f̶.̶i̶t̶e̶m̶ ̶=̶ ̶i̶t̶e̶m̶
    def to_representation(self, instance):
        ...
        do_something_with(s̶e̶l̶f̶.̶i̶t̶e̶m̶ instance)
        ...
class ItemWaveformSerializer(ItemSerializer):
    class Meta:
        model = ts.models.Item
    waveform = WaveformSerializer(I̶T̶E̶M̶_̶N̶E̶E̶D̶E̶D̶)

---

instanceargument of to_representation is enough, nested field need further arguments, see source doc:

waveform = WaveformSerializer(
        source='*',
        many=False,
        read_only=True,
        )

[gnuletik]

Thanks for this @Tointoin !

However, it seems that the schema export the min_values and max_values (from the WaveformSerializer). Should it be replaced with min and max ?