[QUESTION] What is a cannonical representation for SubmodelElementLists with property value = null?

[empwilli]

According to meta model, submodel element lists may have a value = null, as value has a cardinality of 0..* (https://industrialdigitaltwin.io/aas-specifications/IDTA-01001/v3.1/spec-metamodel/submodel-elements.html#submodel-element-list-attributes) and the spec additionally forbids e.g. empty json lists. Same holds for XML.

This raises two questions:

1. For Submodel Instances: Consider a SMT with an optional SubmodelElementList, that may be empty, i.e., a SubmodelElementList with Qualifier SMT/Cardinality of value ZeroToOne with list elements with Qualifier SMT/Cardinality of value ZeroToMany.

If in my Submodel Instance this list is empty. Should I skip the SubmodelElementList element altogether from my submodel, or should I include it, but set its "value" to null?

2. For Submodel Templates: given, that SubmodelElementLists include significant descriptive information about its item type (valueTypeListElement, typeValueListElement, semanticIdListElement), do I always have to specify a list item in my SMT or not? If I not, is there a "default" SMT/Cardinality qualifier supposed for the items?

[seicke]

Using aas4j (for example) to generate a SM including a SML "without" value yields:

<aas:submodelElementList>
   <aas:idShort>harting_sme_smlTest</aas:idShort>
   <aas:orderRelevant>false</aas:orderRelevant>
   <aas:typeValueListElement>Property</aas:typeValueListElement>
</aas:submodelElementList>

{
    "modelType" : "SubmodelElementList",
    "idShort" : "sme_smlTest",
    "orderRelevant" : false,
    "typeValueListElement" : "Property"
}

Regarding question 1: IMHO an optional SML (SMT/Cardinality = ZeroToOne) without any SMEs (value= null, resp. empty array) can be safely skipped. But, if you like, you can add the empty SML to your SM instance (see the two sample code blocks above). To my knowledge, there is no clear guideline here, which, in my opinion, is not absolutely necessary.

Regarding question 2: I'm not sure I understand you correctly, but in a SMT specification including a SML (whether if SMT/Cardinality of the SML is ZeroToOne or One) there should always be a specification of the list items (also whether if SMT/Cardinality of the SME is ZeroToOne or One.

[empwilli]

Thanks for your reply. This mostly aligns with what I have seen and would have interpreted so far.

Regarding question 1: IMHO an optional SML (SMT/Cardinality = ZeroToOne) without any SMEs (value= null, resp. empty array) can be safely skipped. But, if you like, you can add the empty SML to your SM instance (see the two sample code blocks above). To my knowledge, there is no clear guideline here, which, in my opinion, is not absolutely necessary.

The more I think about this, the more I feel like there should be a guideline for implementors, though; especially when different implementations handle empty lists differently. Consider the following situation (admittedly contrived, but even the spec mentioned nested SMLs):

You have an SMT with a list definition:

{
    "modelType": "SubmodelElementList",
    "typeValueListElement": "SubmodelElementList",
    "qualifiers": [{
        "type": "SMT/Cardinality",
        "valueType": "xs:string",
        "value": "ZeroToOne"
    }],
    "value": [{
        "modelType": "SubmodelElementList",
        "typeValueListElement": "Property",
        "valueTypeListElement": "xs:string",
        "qualifiers": [{
            "type": "SMT/Cardinality",
            "valueType": "xs:string",
            "value": "OneToMany"
        }],
        "value": [{
            "modelType": "SubmodelElementList",
            "valueType": "xs:string",
            "qualifiers": [{
                "type": "SMT/Cardinality",
                "valueType": "xs:string",
                "value": "ZeroToMany"
            }]
        }]
    }]
}

In this situation the decision whether an element should be elided if empty or not has a direct effect on the list position of subsequent elements. However, this also means the implementation needs to adjust any model references just for import/export.

[SAidta]

@BirgitBoss what´s your opinion?