DATAREST-944 - DomainObjectMerger now also merges associations for PUT.

We now explicitly merge associations skipping the linkable ones. We also try to reuse the existing collections and maps if they're mutable falling back to a completely new one if not.

Extracted PropertyHandler and AssociationHandler implementations.

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/json/DomainObjectReaderUnitTests.java

@@ -22,6 +22,7 @@ import static org.mockito.Mockito.*;
 import lombok.AllArgsConstructor;
 import lombok.EqualsAndHashCode;
 import lombok.NoArgsConstructor;
+import lombok.Value;
 
 import java.io.ByteArrayInputStream;
 import java.util.ArrayList;
@@ -45,6 +46,7 @@ import org.mockito.runners.MockitoJUnitRunner;
 import org.springframework.data.annotation.CreatedDate;
 import org.springframework.data.annotation.Id;
 import org.springframework.data.annotation.ReadOnlyProperty;
+import org.springframework.data.annotation.Reference;
 import org.springframework.data.annotation.Transient;
 import org.springframework.data.annotation.Version;
 import org.springframework.data.keyvalue.core.mapping.context.KeyValueMappingContext;
@@ -94,6 +96,7 @@ public class DomainObjectReaderUnitTests {
 		mappingContext.getPersistentEntity(Product.class);
 		mappingContext.getPersistentEntity(TransientReadOnlyProperty.class);
 		mappingContext.getPersistentEntity(CollectionOfEnumWithMethods.class);
+		mappingContext.getPersistentEntity(SampleWithReference.class);
 		mappingContext.afterPropertiesSet();
 
 		PersistentEntities entities = new PersistentEntities(Collections.singleton(mappingContext));
@@ -466,6 +469,33 @@ public class DomainObjectReaderUnitTests {
 		assertThat(result.enums, contains(SampleEnum.SECOND, SampleEnum.FIRST));
 	}
 
+	@Test // DATAREST-944
+	public void mergesAssociations() {
+
+		List<Nested> originalCollection = Arrays.asList(new Nested(2, 3));
+		SampleWithReference source = new SampleWithReference(Arrays.asList(new Nested(1, 2), new Nested(2, 3)));
+		SampleWithReference target = new SampleWithReference(originalCollection);
+
+		SampleWithReference result = reader.mergeForPut(source, target, new ObjectMapper());
+
+		assertThat(result.nested, is(source.nested));
+		assertThat(result.nested == originalCollection, is(false));
+	}
+
+	@Test // DATAREST-944
+	public void mergesAssociationsAndKeepsMutableCollection() {
+
+		ArrayList<Nested> originalCollection = new ArrayList<Nested>(Arrays.asList(new Nested(2, 3)));
+		SampleWithReference source = new SampleWithReference(
+				new ArrayList<Nested>(Arrays.asList(new Nested(1, 2), new Nested(2, 3))));
+		SampleWithReference target = new SampleWithReference(originalCollection);
+
+		SampleWithReference result = reader.mergeForPut(source, target, new ObjectMapper());
+
+		assertThat(result.nested, is(source.nested));
+		assertThat(result.nested == originalCollection, is(true));
+	}
+
 	@SuppressWarnings("unchecked")
 	private static <T> T as(Object source, Class<T> type) {
 
@@ -629,4 +659,14 @@ public class DomainObjectReaderUnitTests {
 	static class CollectionOfEnumWithMethods {
 		List<SampleEnum> enums = new ArrayList<SampleEnum>();
 	}
+
+	@Value
+	static class SampleWithReference {
+		@Reference List<Nested> nested;
+	}
+
+	@Value
+	static class Nested {
+		int x, y;
+	}
 }