Flatter relationships

I am thinking of returning json according to the json:api specification for a scientific database / search engine (for mathematicians). My question is about the necessity of the extra “data” layer in the “relationships”

I would somehow like to have

{
  "type": "articles",
  "id": "1",
  "attributes": {
    "title": "Rails is Omakase"
  },
  "relationships": {
    "author": { "type": "people", "id": "9" }
  }
}

More precisely: can the value of a “relationships” member be a “Resource Linkage” object or do I absolutely have to add the additional indirection using “data”?

Alternatively, should I make “author” an attribute?