ToOne and ToMany should allow for having just one of `data`, `links` or `meta` fields about spec HOT 3 CLOSED

"at least one of the following" does mean that it can have just one of data, links and meta.

from spec.

I must apologise - I completely misread this issue when you opened it! I hadn't taken in it was in the laravel-json-api/spec repo.

So the part of the spec that is relevant here says this:

[When creating a resource] If a relationship is provided in the relationships member of the resource object, its value MUST be a relationship object with a data member. The value of this key represents the linkage the new resource is to have.
https://jsonapi.org/format/#crud-creating

And for updating:

If a relationship is provided in the relationships member of a resource object in a PATCH request, its value MUST be a relationship object with a data member. The relationship’s value will be replaced with the value specified in this member.
https://jsonapi.org/format/#crud-updating

The section you've quoted is about the serialization of a relationship within a resource object, i.e. when a server is serializing a resource.

from spec.

I'm sorry, you are complete correct... Guess now I'll have to see how I get my client lib to not serialize relationships for PATCH/POST when there is no data available (but links is)...

from spec.