refactor: Avoid recreation of imperative nested statemachine in some comes.

Fixes #2882

Author: michael-simons

src/main/java/org/springframework/data/neo4j/core/Neo4jTemplate.java

@@ -568,12 +568,14 @@ class Tuple3<T> {
 				.collect(Collectors.toMap(m -> (Value) m.getKey(), m -> (String) m.getValue()));
 
 		// Save related
+		var stateMachine = new NestedRelationshipProcessingStateMachine(neo4jMappingContext, null, null);
 		return entitiesToBeSaved.stream().map(t -> {
 			PersistentPropertyAccessor<T> propertyAccessor = entityMetaData.getPropertyAccessor(t.modifiedInstance);
 			Neo4jPersistentProperty idProperty = entityMetaData.getRequiredIdProperty();
 			Object id = convertIdValues(idProperty, propertyAccessor.getProperty(idProperty));
 			String internalId = idToInternalIdMapping.get(id);
-			return this.<T>processRelations(entityMetaData, propertyAccessor, t.wasNew, new NestedRelationshipProcessingStateMachine(neo4jMappingContext, t.originalInstance, internalId), TemplateSupport.computeIncludePropertyPredicate(pps, entityMetaData));
+			stateMachine.registerInitialObject(t.originalInstance, internalId);
+			return this.<T>processRelations(entityMetaData, propertyAccessor, t.wasNew, stateMachine, TemplateSupport.computeIncludePropertyPredicate(pps, entityMetaData));
 		}).collect(Collectors.toList());
 	}
 

src/main/java/org/springframework/data/neo4j/core/mapping/NestedRelationshipProcessingStateMachine.java

@@ -73,9 +73,15 @@ public NestedRelationshipProcessingStateMachine(final Neo4jMappingContext mappin
 		this.mappingContext = mappingContext;
 	}
 
-	public NestedRelationshipProcessingStateMachine(final Neo4jMappingContext mappingContext, Object initialObject, Object elementId) {
+	public NestedRelationshipProcessingStateMachine(final Neo4jMappingContext mappingContext, @Nullable Object initialObject, @Nullable Object elementId) {
 		this(mappingContext);
 
+		if (initialObject != null && elementId != null) {
+			registerInitialObject(initialObject, elementId);
+		}
+	}
+
+	public void registerInitialObject(Object initialObject, Object elementId) {
 		Assert.notNull(initialObject, "Initial object must not be null");
 		Assert.notNull(elementId, "The initial objects element ID must not be null");