rdf prefix is not emitted when saving to turtle when only rdf:type is used

[cmungall]

whenusing rdflib 6.1.1 to convert the following jsonld to turtle:

{
  "slots": [
    {
      "name": "type",
      "slot_uri": "http://www.w3.org/1999/02/22-rdf-syntax-ns#type"
    }
  ],
  "@context": [
  {
      "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
      "@vocab": "https://w3id.org/linkml/",
      "slot_uri": {
         "@type": "@id"
      }
    }
  ]
}

the turtle emitted is:

@prefix : <https://w3id.org/linkml/> .

[] :slots [ :name "type" ;
            :slot_uri rdf:type ] .

which is invalid, as it lacks a prefix declaration for rdf

The situation is resolved by either

• adding another node to the graph in the rdf namespace, other than rdf:type
• saving to a format like nt

This reproduces the error:

JSONLD = """
{
  "slots": [
    {
      "name": "type",
      "slot_uri": "http://www.w3.org/1999/02/22-rdf-syntax-ns#type"
    }
  ],
  "@context": [
  {
      "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
      "@vocab": "https://w3id.org/linkml/",
      "slot_uri": {
         "@type": "@id"
      }
    }
  ]
}"""

from rdflib import Graph

graph = Graph()
graph.parse(data=JSONLD, format="json-ld", prefix=False)
ttl_str = graph.serialize(format='turtle')
print(ttl_str)
graph.parse(data=ttl_str, format="turtle")

This is in contrast to other libs, e.g. Jena, which will produce valid turtle:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .

_:b0    <https://w3id.org/linkml/slots>  _:b1 .

_:b1    <https://w3id.org/linkml/name>  "type" ;
        <https://w3id.org/linkml/slot_uri>  rdf:type .

[nicholascar]

I think that the use of rdf:type as an object, as opposed to the commonly seen predicate, is somehow confusing RDFlib. This use is perfectly valid, so this is an RDFLib bug. Tagging as such.

RDFLib will always convert rdf:type to a if it's used as a predicate, so that why RDFLib thinks it doesn't need to add the rdf: prefix, but it's wrong!