How to refer to RDF statements in JSON-LD? How to state(ment) about statements?

Question

Suppose you have a simple RDF statement x:Object x:predicate x:Subject, which is expressed in JSON-LD as {"@id": "x:Object", "x:predicate": {"@id": "x:Subject"}}. How do you refer to the specific object-predicate-subject relation, how do you "adress" the factual relation? What's the "@id" or the inline identity of a statement in JSON-LD? How do you ascribe "meta data" to a statement?

How do you express statements in JSON-LD, where the subject is another statement, e.g. [x:Subject x:predicate x:Object] x:metaPredicate x:MetaObject?

How do you express statements in JSON-LD, where the object is another statement, e.g. x:MetaSubject x:metaPredicate [x:Subject x:predicate x:Object]?

How do you express statements in JSON-LD, where the predicate is another statement (strange but possible), e.g. x:MetaSubject [x:Subject x:predicate x:Object] x:MetaObject?

(PS: I realize the [] syntax of my samples is not conformant Turtle, but they serve to express my thoughts/questions.)

Answer 1

You need to use reification, there exists a standardized vocabulary for that. In JSON-LD it would look somewhat like this:

{
  "@context": {
    "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
    "subject": { "@id": "rdf:subject", "@type": "@id" },
    "predicate": { "@id": "rdf:predicate", "@type": "@id" },
    "object": { "@id": "rdf:object", "@type": "@id" }
  },
  "@type": "rdf:Statement",
  "subject": "x:Subject",
  "predicate": "x:predicate",
  "object": { "@id": "x:Object" },
  "x:metaPredicate": "x:MetaObject"
}

Answer 2

One may now be able to reference an RDF triple in JSON-LD by using JSON-LD*, or JSON-LD-star.

Referencing a triple as the subject

JSON-LD* builds on top of RDF*, which is already supported by some tooling, and makes it possible to use an RDF triple in place of the subject, e.g.:

<< s pred o >> pred2 o2

in this way:

{
  "@id": {"@id": "s", "pred": {"@id": "o"}},
  "pred2": {"@id": "o2" }
}

Referencing a triple as the object

It should be noted that the JSON-LD structure for using an RDF triple as an object is a bit awkward, and this:

s pred << s2 pred2 o >>

requires a @reverse, expressed like this:

{
  "@id": {
    "@id": "s2",
    "pred2": {"@id": "o"}
  },
  "@reverse": {
    "pred": {"@id": "s"}
  }
}

See also

• W3C community group JSON-LD-star spec draft