Populate DTO projection properties that are considered associations.

We now populate DTO properties whose are identified as associations. While uncommon, as typically entities declare assocations using entities, associations can be identified as types and so these are now considered when populating properties.

Closes #3104

Author: mp911de

Diff for: src/main/java/org/springframework/data/convert/DtoInstantiatingConverter.java

@@ -21,6 +21,7 @@
 import org.springframework.data.mapping.PersistentEntity;
 import org.springframework.data.mapping.PersistentProperty;
 import org.springframework.data.mapping.PersistentPropertyAccessor;
+import org.springframework.data.mapping.SimpleAssociationHandler;
 import org.springframework.data.mapping.SimplePropertyHandler;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mapping.model.EntityInstantiator;
@@ -96,15 +97,19 @@ public Object getParameterValue(Parameter parameter) {
 		PersistentPropertyAccessor<Object> targetAccessor = targetEntity.getPropertyAccessor(dto);
 		InstanceCreatorMetadata<? extends PersistentProperty<?>> creator = targetEntity.getInstanceCreatorMetadata();
 
-		targetEntity.doWithProperties((SimplePropertyHandler) property -> {
+		SimplePropertyHandler simplePropertyHandler = property -> {
 
 			if ((creator != null) && creator.isCreatorParameter(property)) {
 				return;
 			}
 
 			targetAccessor.setProperty(property,
 					sourceAccessor.getProperty(sourceEntity.getRequiredPersistentProperty(property.getName())));
-		});
+		};
+
+		targetEntity.doWithProperties(simplePropertyHandler);
+		targetEntity.doWithAssociations(
+				(SimpleAssociationHandler) property -> simplePropertyHandler.doWithPersistentProperty(property.getInverse()));
 
 		return dto;
 	}

Diff for: src/test/java/org/springframework/data/convert/DtoInstantiatingConverterUnitTests.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 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.convert;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.data.annotation.Reference;
+import org.springframework.data.mapping.context.SampleMappingContext;
+import org.springframework.data.mapping.model.EntityInstantiators;
+
+/**
+ * Unit tests for {@link DtoInstantiatingConverter}.
+ *
+ * @author Mark Paluch
+ */
+class DtoInstantiatingConverterUnitTests {
+
+	@Test // GH- 3104
+	void dtoProjectionShouldConsiderPropertiesAndAssociations() {
+
+		TheOtherThing ref = new TheOtherThing();
+		MyAssociativeEntity entity = new MyAssociativeEntity("1", ref, "foo");
+		DtoInstantiatingConverter converter = new DtoInstantiatingConverter(DtoProjection.class, new SampleMappingContext(),
+				new EntityInstantiators());
+
+		DtoProjection projection = (DtoProjection) converter.convert(entity);
+
+		assertThat(projection.id).isEqualTo("1");
+		assertThat(projection.ref).isSameAs(ref);
+	}
+
+	static class MyAssociativeEntity {
+
+		String id;
+
+		@Reference TheOtherThing ref;
+
+		String somethingElse;
+
+		public MyAssociativeEntity(String id, TheOtherThing ref, String somethingElse) {
+			this.id = id;
+			this.ref = ref;
+			this.somethingElse = somethingElse;
+		}
+	}
+
+	static class DtoProjection {
+
+		String id;
+
+		@Reference TheOtherThing ref;
+	}
+
+	static class TheOtherThing {
+		String id;
+	}
+
+}