ToOne and ToMany should allow for having just one of data, links or meta fields

[yaviso]

Per the spec:

A “relationship object” MUST contain at least one of the following:

• links: a links object containing at least one of the following:
  • self: a link for the relationship itself (a “relationship link”). This link allows the client to directly manipulate the relationship. > For example, removing an author through an article’s relationship URL would disconnect the person from the article without deleting the people resource itself. When fetched successfully, this link returns the linkage for the related resources as its primary data. (See Fetching Relationships.)
  • related: a related resource link
• data: resource linkage
• meta: a meta object that contains non-standard meta-information about the relationship.

[lindyhopchris]

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

[lindyhopchris]

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.

[yaviso]

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)...