DATAREST-327 - DomainObjectMerger doesn't merge empty collections.

The association handling in DomainObjectMerger is in place to allow the creation of resources using PUT that have non-optional associations. However, if the domain types that the request payload was unmarshalled into use default values - in particular empty collections to avoid nulls - the merger cannot really distinguish between the default set in the type versus an empty collection being submitted through the request.

As the usecase here is creation only we can safely ignore empty collections as submitting those doesn't make a difference anyway (no related items attached). By ignoring those, we fix the issue defaulted empty collections in the type definitions being considered as value to set.

spring-data-rest-core/src/main/java/org/springframework/data/rest/core/support/DomainObjectMerger.java

@@ -112,13 +112,37 @@ public class DomainObjectMerger {
 				PersistentProperty<?> persistentProperty = association.getInverse();
 				Object fromVal = fromWrapper.getProperty(persistentProperty);
 
-				if (fromVal != null && !fromVal.equals(targetWrapper.getProperty(persistentProperty))) {
+				if (!isNullOrEmpty(fromVal) && !fromVal.equals(targetWrapper.getProperty(persistentProperty))) {
 					targetWrapper.setProperty(persistentProperty, fromVal);
 				}
 			}
 		});
 	}
 
+	/**
+	 * Returns whether the given source is {@literal null} or considered empty, which means it's an {@link Iterable} or
+	 * array and doesn't have any elements.
+	 * 
+	 * @param source can be {@literal null}.
+	 * @return
+	 */
+	static boolean isNullOrEmpty(Object source) {
+
+		if (source == null) {
+			return true;
+		}
+
+		if (source instanceof Iterable) {
+			return !((Iterable<?>) source).iterator().hasNext();
+		}
+
+		if (ObjectUtils.isArray(source)) {
+			return ObjectUtils.isEmpty((Object[]) source);
+		}
+
+		return false;
+	}
+
 	/**
 	 * Strategy to express whether {@literal null} values should be ignored or set on the target domain object.
 	 */

spring-data-rest-core/src/test/java/org/springframework/data/rest/core/support/DomainObjectMergerTests.java

@@ -19,16 +19,17 @@ import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 import static org.springframework.data.rest.core.support.DomainObjectMerger.NullHandlingPolicy.*;
 
+import java.util.Arrays;
+
+import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.context.ConfigurableApplicationContext;
-import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.data.repository.support.Repositories;
 import org.springframework.data.rest.core.domain.jpa.JpaRepositoryConfig;
 import org.springframework.data.rest.core.domain.jpa.Person;
-import org.springframework.data.rest.core.domain.jpa.PersonRepository;
 import org.springframework.test.context.ContextConfiguration;
 import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
 
@@ -42,26 +43,28 @@ import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
 @ContextConfiguration(classes = JpaRepositoryConfig.class)
 public class DomainObjectMergerTests {
 
-	@Autowired PersonRepository personRepository;
 	@Autowired ConfigurableApplicationContext context;
 
+	DomainObjectMerger merger;
+
+	@Before
+	public void setUp() {
+		this.merger = new DomainObjectMerger(new Repositories(context.getBeanFactory()), new DefaultConversionService());
+	}
+
 	/**
 	 * @see DATAREST-130
 	 */
 	@Test
 	public void mergeNewValue() {
 
-		Repositories repositories = new Repositories(context.getBeanFactory());
-		ConversionService conversionService = new DefaultConversionService();
-
 		Person incoming = new Person("Bilbo", "Baggins");
 		Person existingDomainObject = new Person("Frodo", "Baggins");
 
-		DomainObjectMerger merger = new DomainObjectMerger(repositories, conversionService);
 		merger.merge(incoming, existingDomainObject, APPLY_NULLS);
 
-		assertThat(existingDomainObject.getFirstName(), equalTo(incoming.getFirstName()));
-		assertThat(existingDomainObject.getLastName(), equalTo(incoming.getLastName()));
+		assertThat(existingDomainObject.getFirstName(), is(incoming.getFirstName()));
+		assertThat(existingDomainObject.getLastName(), is(incoming.getLastName()));
 	}
 
 	/**
@@ -70,16 +73,28 @@ public class DomainObjectMergerTests {
 	@Test
 	public void mergeNullValue() {
 
-		Repositories repositories = new Repositories(context.getBeanFactory());
-		ConversionService conversionService = new DefaultConversionService();
-
 		Person incoming = new Person(null, null);
 		Person existingDomainObject = new Person("Frodo", "Baggins");
 
-		DomainObjectMerger merger = new DomainObjectMerger(repositories, conversionService);
 		merger.merge(incoming, existingDomainObject, APPLY_NULLS);
 
-		assertThat(existingDomainObject.getFirstName(), equalTo(incoming.getFirstName()));
-		assertThat(existingDomainObject.getLastName(), equalTo(incoming.getLastName()));
+		assertThat(existingDomainObject.getFirstName(), is(incoming.getFirstName()));
+		assertThat(existingDomainObject.getLastName(), is(incoming.getLastName()));
+	}
+
+	/**
+	 * @see DATAREST-327
+	 */
+	@Test
+	public void doesNotMergeEmptyCollectionsForReferences() {
+
+		Person bilbo = new Person("Bilbo", "Baggins");
+
+		Person frodo = new Person("Frodo", "Baggins");
+		frodo.setSiblings(Arrays.asList(bilbo));
+
+		merger.merge(new Person("Sam", null), frodo, IGNORE_NULLS);
+
+		assertThat(frodo.getSiblings(), is(not(emptyIterable())));
 	}
 }

spring-data-rest-core/src/test/java/org/springframework/data/rest/core/support/DomainObjectMergerUnitTests.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2014 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
+ *
+ *      http://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.rest.core.support;
+
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+import static org.springframework.data.rest.core.support.DomainObjectMerger.*;
+
+import java.util.Collections;
+import java.util.Iterator;
+
+import org.junit.Test;
+
+/**
+ * Unit tests for {@link DomainObjectMerger}.
+ * 
+ * @author Oliver Gierke
+ */
+public class DomainObjectMergerUnitTests {
+
+	/**
+	 * @see DATAREST-327
+	 */
+	@Test
+	public void considersEmptyObjectsEmpty() {
+
+		assertThat(isNullOrEmpty(null), is(true));
+		assertThat(isNullOrEmpty(Collections.emptyList()), is(true));
+		assertThat(isNullOrEmpty(new Object[0]), is(true));
+		assertThat(isNullOrEmpty(new String[0]), is(true));
+		assertThat(isNullOrEmpty(new MyIterable()), is(true));
+
+		assertThat(isNullOrEmpty(new Object()), is(false));
+		assertThat(isNullOrEmpty(Collections.singleton(new Object())), is(false));
+		assertThat(isNullOrEmpty(new Object[] { "1" }), is(false));
+		assertThat(isNullOrEmpty(new String[] { "1" }), is(false));
+	}
+
+	class MyIterable implements Iterable<Object> {
+
+		@Override
+		public Iterator<Object> iterator() {
+			return Collections.emptyList().iterator();
+		}
+	}
+}