Not an array when only one temporal instance of a given attribute

[EliottPaillard]

Hello, I'm using the stellio images version 2.12.3

I noticed something not aligned with the NGSI-LD Specifications (1.8.1)

When trying to retrieve a temporal representation of an entity, an attribute which has only one instance in this time interval is represented by a JSON object and not an array of object.

The expected behaviour is described in part 4.5.7 of upper said document: "The temporal evolution of an NGSI-LD Property shall be represented as an Array of JSON-LD objects, each one representing an instance of the Property"

[bobeal]

Hi @EliottPaillard,

Thanks for reporting this.

This behavior is actually due to the JSON-LD compaction algorithm that flattens the arrays of one object into an object. I guess all NGSI-LD brokers have this same behavior... I will raise this point to ISG-CIM to see how to handle it (whether brokers should handle this "manually" or an "exception" should be added in the specification for this specific case).

[bobeal]

We discussed about this with ISG-CIM. This is indeed normal behavior of JSON-LD compaction (as stated in section 5.2.7 in https://www.w3.org/TR/json-ld11/#representing-singular-values-as-arrays). Hence, JSON-LD arrays (used for the temporal representation of attributes) comprising just one element are returned as a single element, not an array with just one element in it.

A note clarifying this behavior will be added in the next version of the specification.

[bobeal]

FYI, we discussed again about this with ISG-CIM. As there are other cases where single objects are forced to be wrapped as arrays, we finally decided that such temporal instances should always be returned as arrays (even if there is only one element and if it is kind of overwriting of normal JSON-LD compaction).

[EliottPaillard]

Thank you for this