Allow optional first/second in relationship elements

sdk/basyx/aas/adapter/json/json_deserialization.py

@@ -635,8 +635,8 @@ def _construct_operation(cls, dct: Dict[str, object], object_class=model.Operati
     def _construct_relationship_element(
             cls, dct: Dict[str, object], object_class=model.RelationshipElement) -> model.RelationshipElement:
         ret = object_class(id_short=None,
-                           first=cls._construct_reference(_get_ts(dct, 'first', dict)),
-                           second=cls._construct_reference(_get_ts(dct, 'second', dict)))
+                           first=cls._construct_reference(_get_ts(dct, 'first', dict)) if 'first' in dct else None,
+                           second=cls._construct_reference(_get_ts(dct, 'second', dict)) if 'second' in dct else None)
         cls._amend_abstract_attributes(ret, dct)
         return ret
 
@@ -646,8 +646,8 @@ def _construct_annotated_relationship_element(
             -> model.AnnotatedRelationshipElement:
         ret = object_class(
             id_short=None,
-            first=cls._construct_reference(_get_ts(dct, 'first', dict)),
-            second=cls._construct_reference(_get_ts(dct, 'second', dict)))
+            first=cls._construct_reference(_get_ts(dct, 'first', dict)) if 'first' in dct else None,
+            second=cls._construct_reference(_get_ts(dct, 'second', dict)) if 'second' in dct else None)
         cls._amend_abstract_attributes(ret, dct)
         if not cls.stripped and 'annotations' in dct:
             for element in _get_ts(dct, 'annotations', list):

sdk/basyx/aas/adapter/json/json_serialization.py

@@ -567,7 +567,10 @@ def _relationship_element_to_json(cls, obj: model.RelationshipElement) -> Dict[s
         :return: dict with the serialized attributes of this object
         """
         data = cls._abstract_classes_to_json(obj)
-        data.update({'first': obj.first, 'second': obj.second})
+        if obj.first is not None:
+            data.update({'first': obj.first})
+        if obj.second is not None:
+            data.update({'second': obj.second})
         return data
 
     @classmethod

sdk/basyx/aas/adapter/xml/xml_deserialization.py

@@ -512,8 +512,8 @@ def _construct_relationship_element_internal(cls, element: etree._Element, objec
         """
         relationship_element = object_class(
             None,
-            _child_construct_mandatory(element, NS_AAS + "first", cls.construct_reference),
-            _child_construct_mandatory(element, NS_AAS + "second", cls.construct_reference)
+            _failsafe_construct(element.find(NS_AAS + "first"), cls.construct_reference, cls.failsafe),
+            _failsafe_construct(element.find(NS_AAS + "second"), cls.construct_reference, cls.failsafe)
         )
         cls._amend_abstract_attributes(relationship_element, element)
         return relationship_element

sdk/basyx/aas/model/submodel.py

@@ -904,8 +904,8 @@ class AnnotatedRelationshipElement(RelationshipElement, base.UniqueIdShortNamesp
 
     def __init__(self,
                  id_short: Optional[base.NameType],
-                 first: base.Reference,
-                 second: base.Reference,
+                 first: Optional[base.Reference] = None,
+                 second: Optional[base.Reference] = None,
                  display_name: Optional[base.MultiLanguageNameType] = None,
                  annotation: Iterable[DataElement] = (),
                  category: Optional[base.NameType] = None,

sdk/test/adapter/json/test_json_deserialization.py

@@ -329,6 +329,31 @@ def test_stripped_annotated_relationship_element(self) -> None:
         assert isinstance(are, model.AnnotatedRelationshipElement)
         self.assertEqual(len(are.annotation), 0)
 
+    def test_optional_first_second_relationship_element(self) -> None:
+        data = """
+            {
+                "modelType": "RelationshipElement",
+                "idShort": "test_optional_second_relationship_element",
+                "category": "PARAMETER",
+                "first": {
+                    "type": "ModelReference",
+                    "keys": [
+                        {
+                            "type": "Submodel",
+                            "value": "http://example.org/Test_Submodel"
+                        },
+                        {
+                            "type": "AnnotatedRelationshipElement",
+                            "value": "test_ref"
+                        }
+                    ]
+                }
+            }"""
+
+        re = json.loads(data, cls=StrictAASFromJsonDecoder)
+        self.assertIsInstance(re, model.RelationshipElement)
+        self.assertIsNone(re.second)
+
     def test_stripped_entity(self) -> None:
         data = """
             {

sdk/test/adapter/json/test_json_serialization.py

@@ -208,6 +208,12 @@ def test_stripped_annotated_relationship_element(self) -> None:
 
         self._checkNormalAndStripped("annotations", are)
 
+    def test_relationship_element_omits_none_first_second(self) -> None:
+        re = model.RelationshipElement("test_re")
+        data = json.loads(json.dumps(re, cls=AASToJsonEncoder))
+        self.assertNotIn("first", data)
+        self.assertNotIn("second", data)
+
     def test_stripped_entity(self) -> None:
         mlp = model.MultiLanguageProperty("test_multi_language_property", category="PARAMETER")
         entity = model.Entity("test_entity", model.EntityType.CO_MANAGED_ENTITY, statement=[mlp])

sdk/test/adapter/xml/test_xml_deserialization.py

@@ -471,6 +471,41 @@ def test_data_spec_iec61360_value_without_value_format(self) -> None:
         self.assertEqual("test_value", ds_content.value)
         self.assertIsNone(ds_content.value_format)
 
+    def test_optional_first_second_relationship_element(self) -> None:
+        xml = _xml_wrap("""
+        <aas:submodels>
+            <aas:submodel>
+                <aas:id>http://example.org/test_submodel</aas:id>
+                <aas:submodelElements>
+                    <aas:relationshipElement>
+                        <aas:idShort>test_optional_second_relationship_element</aas:idShort>
+                        <aas:category>PARAMETER</aas:category>
+                        <aas:first>
+                            <aas:type>ModelReference</aas:type>
+                            <aas:keys>
+                                <aas:key>
+                                    <aas:type>Submodel</aas:type>
+                                    <aas:value>http://example.org/Test_Submodel</aas:value>
+                                </aas:key>
+                                <aas:key>
+                                    <aas:type>AnnotatedRelationshipElement</aas:type>
+                                    <aas:value>test_ref</aas:value>
+                                </aas:key>
+                            </aas:keys>
+                        </aas:first>
+                    </aas:relationshipElement>
+                </aas:submodelElements>
+            </aas:submodel>
+        </aas:submodels>
+        """)
+        object_store = read_aas_xml_file(io.StringIO(xml), failsafe=False)
+        submodel = object_store.get_item("http://example.org/test_submodel")
+        assert isinstance(submodel, model.Submodel)
+        re = submodel.get_referable("test_optional_second_relationship_element")
+        self.assertIsInstance(re, model.RelationshipElement)
+        assert isinstance(re, model.RelationshipElement)
+        self.assertIsNone(re.second)
+
 
 class XmlDeserializationDerivingTest(unittest.TestCase):
     def test_submodel_constructor_overriding(self) -> None: