fix: Take generic field type into account for @RelationshipProperties.

This falls back to checking the field type, so that generic relationship properties can be used instead of a concrete implementation.

Closes #2911.

Author: michael-simons

src/main/java/org/springframework/data/neo4j/core/mapping/DefaultNeo4jPersistentProperty.java

@@ -16,9 +16,11 @@
 package org.springframework.data.neo4j.core.mapping;
 
 import java.lang.reflect.Field;
+import java.lang.reflect.ParameterizedType;
 import java.util.Collections;
 import java.util.Optional;
 
+import org.springframework.core.ResolvableType;
 import org.springframework.data.annotation.ReadOnlyProperty;
 import org.springframework.data.mapping.Association;
 import org.springframework.data.mapping.MappingException;
@@ -117,8 +119,8 @@ protected Association<Neo4jPersistentProperty> createAssociation() {
 		Neo4jPersistentEntity<?> relationshipPropertiesClass = null;
 
 		if (this.hasActualTypeAnnotation(RelationshipProperties.class)) {
-			Class<?> type = getRelationshipPropertiesTargetType(getActualType());
-			obverseOwner = this.mappingContext.addPersistentEntity(TypeInformation.of(type)).get();
+			TypeInformation<?> typeInformation = getRelationshipPropertiesTargetType(getActualType());
+			obverseOwner = this.mappingContext.addPersistentEntity(typeInformation).get();
 			relationshipPropertiesClass = this.mappingContext.addPersistentEntity(TypeInformation.of(getActualType())).get();
 		} else {
 			Class<?> associationTargetType = this.getAssociationTargetType();
@@ -136,8 +138,8 @@ protected Association<Neo4jPersistentProperty> createAssociation() {
 						mapValueType.getType().isAnnotationPresent(RelationshipProperties.class);
 
 				if (relationshipPropertiesCollection) {
-					Class<?> type = getRelationshipPropertiesTargetType(mapValueType.getActualType().getType());
-					obverseOwner = this.mappingContext.addPersistentEntity(TypeInformation.of(type)).get();
+					TypeInformation<?> typeInformation = getRelationshipPropertiesTargetType(mapValueType.getActualType().getType());
+					obverseOwner = this.mappingContext.addPersistentEntity(typeInformation).get();
 					relationshipPropertiesClass = this.mappingContext
 							.addPersistentEntity(mapValueType.getComponentType()).get();
 
@@ -179,7 +181,7 @@ protected Association<Neo4jPersistentProperty> createAssociation() {
 	}
 
 	@NonNull
-	private Class<?> getRelationshipPropertiesTargetType(Class<?> relationshipPropertiesType) {
+	private TypeInformation<?> getRelationshipPropertiesTargetType(Class<?> relationshipPropertiesType) {
 
 		Field targetNodeField = ReflectionUtils.findField(relationshipPropertiesType,
 				field -> field.isAnnotationPresent(TargetNode.class));
@@ -188,7 +190,11 @@ private Class<?> getRelationshipPropertiesTargetType(Class<?> relationshipProper
 			throw new MappingException("Missing @TargetNode declaration in " + relationshipPropertiesType);
 		}
 		TypeInformation<?> relationshipPropertiesTypeInformation = TypeInformation.of(relationshipPropertiesType);
-		return relationshipPropertiesTypeInformation.getProperty(targetNodeField.getName()).getType();
+		Class<?> type = relationshipPropertiesTypeInformation.getProperty(targetNodeField.getName()).getType();
+		if (Object.class == type && this.getField().getGenericType() instanceof ParameterizedType pt && pt.getActualTypeArguments().length == 1) {
+			return TypeInformation.of(ResolvableType.forType(pt.getActualTypeArguments()[0]));
+		}
+		return TypeInformation.of(type);
 	}
 
 	@Override

src/test/java/org/springframework/data/neo4j/core/mapping/Neo4jMappingContextTest.java

@@ -59,6 +59,8 @@
 import org.springframework.data.neo4j.core.mapping.datagraph1446.R2;
 import org.springframework.data.neo4j.core.mapping.datagraph1448.A_S3;
 import org.springframework.data.neo4j.core.mapping.datagraph1448.RelatedThing;
+import org.springframework.data.neo4j.core.mapping.genericRelProperties.Source;
+import org.springframework.data.neo4j.core.mapping.genericRelProperties.Target;
 import org.springframework.data.neo4j.core.mapping.gh2574.Model;
 import org.springframework.data.neo4j.core.schema.CompositeProperty;
 import org.springframework.data.neo4j.core.schema.GeneratedValue;
@@ -682,6 +684,16 @@ void hierarchyMustBeConsistentlyReported() throws ClassNotFoundException {
 		assertThat(children).containsExactly("A2", "A3", "A4");
 	}
 
+	@Test // GH-2911
+	void shouldDealWithGenericRelationshipProperties() {
+		Neo4jMappingContext schema = new Neo4jMappingContext();
+		Neo4jPersistentEntity<?> persistentEntity = schema.getPersistentEntity(Source.class);
+		assertThat(persistentEntity.getRelationships()).hasSize(1).first().satisfies(r -> {
+			var target = r.getTarget();
+			assertThat(target.getUnderlyingClass()).isEqualTo(Target.class);
+		});
+	}
+
 	static class EntityWithPostLoadMethods {
 
 		String m1;

src/test/java/org/springframework/data/neo4j/core/mapping/genericRelProperties/Properties.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2011-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.neo4j.core.mapping.genericRelProperties;
+
+import org.springframework.data.neo4j.core.schema.GeneratedValue;
+import org.springframework.data.neo4j.core.schema.Id;
+import org.springframework.data.neo4j.core.schema.RelationshipProperties;
+import org.springframework.data.neo4j.core.schema.TargetNode;
+
+/**
+ * @author Michael J. Simons
+ * @param <T> The crux of this class
+ */
+@RelationshipProperties
+public class Properties<T> {
+
+	@Id
+	@GeneratedValue
+	String elementId;
+
+	@TargetNode
+	T target;
+}

src/test/java/org/springframework/data/neo4j/core/mapping/genericRelProperties/Source.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2011-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.neo4j.core.mapping.genericRelProperties;
+
+import org.springframework.data.neo4j.core.schema.Relationship;
+
+/**
+ * @author Michael J. Simons
+ */
+public class Source {
+
+	@Relationship("IS_RELATED_TO")
+	private Properties<Target> target;
+}

src/test/java/org/springframework/data/neo4j/core/mapping/genericRelProperties/Target.java

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2011-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.neo4j.core.mapping.genericRelProperties;
+
+/**
+ * @author Michael J. Simons
+ */
+public class Target {
+}