Revision history [back]
 | 1 | initial version |
Your eMail poses some interesting questions which I will attempt to answer. Properties and Relationships are defined within the ETSI specification:
https://www.etsi.org/deliver/etsigs/CIM/001099/009/01.03.0160/gscim009v010301p.pdf
and trying to reduce this 200 page document to a series of bite size tutorials is quite a task. The short answer is yes, one-to-many relationships are fully supported by NGSI-LD, although the official syntax is more verbose than I personally would like.
The 1.3.1 spec supports arrays of relationships as shown:
curl -L -X POST 'http://localhost:1026/ngsi-ld/v1/entities/urn:ngsi-ld:Building:store001/attrs' \ -H 'Content-Type: application/ld+json' \ --data-raw '{ "furniture": [ { "type": "Relationship", "datasetId": "urn:ngsi-ld:Relationship:1", "object": "urn:ngsi-ld:Shelf:001" } ,
{ "type": "Relationship", "datasetId": "urn:ngsi-ld:Relationship:2", "object": "urn:ngsi-ld:Shelf:002" } ], "@context": "https://fiware.github.io/tutorials.Step-by-Step/data-models-context.jsonld" }'
Each element beyond the first must have a unique datasetId. This syntax is also used for Multi valued Properties or GeoProperties (For example where location is being provided by two different sensors e.g. GPS urn:ngsi-ld:GPS or Wifi triangulation. urn:ngsi-ld:WiFi)
The example above comes from the older NGSI-LD for NGSI-v2 Supermarket tutorial https://github.com/FIWARE/tutorials.Relationships-Linked-Data#nine-request I haven't added an equivalent in the newer Smart Farm NGSI-LD first Smart Farm system yet.
The Orion-LD context broker still supports an unofficial simplified syntax:
curl -L -X POST 'http://localhost:1026/ngsi-ld/v1/entities/urn:ngsi-ld:Building:store001/attrs' \ -H 'Content-Type: application/ld+json' \ --data-raw '{ "furniture":
{ "type": "Relationship", "object": [ "urn:ngsi-ld:Shelf:001", "urn:ngsi-ld:Shelf:002"] } , "@context": "https://fiware.github.io/tutorials.Step-by-Step/data-models-context.jsonld" }'
This mimics the key-values format and also allows one-to-many relationships, but does not allow for meta-data on each element.
As you correctly stated, relationships are directional, and this furniture one-to-many is the direct opposite of reusing something like locatedIn which would appear on multiple entities
curl -X POST \ http://localhost:1026/ngsi-ld/v1/entities/urn:ngsi-ld:Shelf:unit001/attrs \ -H 'Content-Type: application/ld+json' \ -H 'fiware-servicepath: /' \ -d '{ "locatedIn" :
{ "type": "Relationship", "object": "urn:ngsi-ld:Building:store001", } , "@context": "https://fiware.github.io/tutorials.Step-by-Step/data-models-context.jsonld" }'
Within NGSI-LD the object attribute of a Relationship is specified to be a URN - it can point to any other object, so there is no problem with your Combined urn-ngsi-ld:Computer:001 and urn-ngsi-ld:Mower:002 example.
