Correctly set up constant fields in API responses

[obulat]

Fixes

Fixes #3034 by @obulat

Description

This PR replaces ReadOnlyFields with SerializerMethodFields that return constant values for values that never change and are not derived from models:
OEmbedSerializer: version, type
ProviderSerializer: logo_url

This makes the documentation a little bit clearer and fixes the warnings in logs.

Currently, logo_url type is given as string, but it always returns null. The updated version shows string or null. I tried setting it to "only null", but spectacular still throws a warning that it couldn't get the type of the value. string or null is more accurate than string.

OEmbed serializer

Currently, the documentation says that these values are "default", making it seem like a different value is also possible.

In this PR, the documentation says "Value: 1.0", making it clear that it's only a single value.

Testing Instructions

If you run the app on main (just up), and open the documentation (e.g. http://0.0.0.0:50280/v1/#tag/audio/operation/audio_stats), you should see the warnings mentioned in the related issue in the Docker logs.
These warnings should not be visible in this PR. The documentation for these fields should be updated to match snapshots in the PR description.

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• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
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• I ran the DAG documentation generator (if applicable).

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[dhruvkb]

While the change to the documentation is good, something feels off about using a "method" field to generate a constant value. One alternative might be to create a custom field but that seems like over-engineering.

Approving because I don't know a better way.

[obulat]

Added args that are discarded (*args) to the get_type and get_version functions to fix TypeError: OembedSerializer.get_version() takes 0 positional arguments but 1 was given error that's causing error in the oembed endpoint: 490095d

[sarayourfriend]

The swagger docs say, regarding "null" (TIL):

Note that there is no null type; instead, the nullable attribute is used as a modifier of the base type.

So a field cannot only be null, it seems, and within spec.

For the static fields, I wrote this custom field that seems to work:

class StaticField(serializers.Field):
    def __new__(cls, value, **kwargs):
        decorated = extend_schema_field({"type": "string", "nullable": True} if value is None else type(value))(cls)
        return super().__new__(decorated, value, **kwargs)

    def __init__(self, value, **kwargs):
        self.value = value
        kwargs["source"] = "*"
        kwargs["read_only"] = True

        super().__init__(**kwargs)

    def to_representation(self):
        return self.value

The __new__ method is necessary to pull the type from the passed value (or set it to a nullable string, if the value is None). extend_schema_field comes from drf_spectacular.utils. Not sure whether it's necessary, the method fields work just fine, but if you want something a little more "magical", there you go 🙂

[sarayourfriend]

LGTM!

[obulat]

but if you want something a little more "magical", there you go 🙂

Thank you for the implementation and sharing the info on nullability 😄 I think we can go with what we have here, and change if needed later, if this starts to cause problems