DATAJPA-931 - Avoid unnecessary merging on save.

Checking if entity is already attached to entity manager before calling merge.

Fixed one test that was relying on the implicit flush triggered by the save.

See also: https://vladmihalcea.com/2016/07/19/jpa-persist-and-merge/

src/main/java/org/springframework/data/jpa/repository/support/SimpleJpaRepository.java

@@ -67,6 +67,7 @@ import org.springframework.util.Assert;
  * @author Thomas Darimont
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Jens Schauder
  * @param <T> the type of the entity to handle
  * @param <ID> the type of the entity's identifier
  */
@@ -488,9 +489,11 @@ public class SimpleJpaRepository<T, ID> implements JpaRepository<T, ID>, JpaSpec
 		if (entityInformation.isNew(entity)) {
 			em.persist(entity);
 			return entity;
-		} else {
+		} else if (!em.contains(entity)) {
 			return em.merge(entity);
 		}
+
+		return entity;
 	}
 
 	/*

src/test/java/org/springframework/data/jpa/domain/support/AuditingEntityListenerTests.java

@@ -41,6 +41,7 @@ import org.springframework.transaction.annotation.Transactional;
  * Integration test for {@link AuditingEntityListener}.
  *
  * @author Oliver Gierke
+ * @author Jens Schauder
  */
 @RunWith(SpringJUnit4ClassRunner.class)
 @ContextConfiguration("classpath:auditing/auditing-entity-listener.xml")
@@ -89,7 +90,9 @@ public class AuditingEntityListenerTests {
 		role.setName("ADMIN");
 
 		user.addRole(role);
-		repository.save(user);
+
+		repository.saveAndFlush(user);
+
 		role = user.getRoles().iterator().next();
 
 		assertDatesSet(user);

src/test/java/org/springframework/data/jpa/repository/support/SimpleJpaRepositoryUnitTests.java

@@ -44,6 +44,7 @@ import org.springframework.data.repository.CrudRepository;
  * @author Oliver Gierke
  * @author Thomas Darimont
  * @author Mark Paluch
+ * @author Jens Schauder
  */
 @RunWith(MockitoJUnitRunner.Silent.class)
 public class SimpleJpaRepositoryUnitTests {
@@ -131,4 +132,28 @@ public class SimpleJpaRepositoryUnitTests {
 
 		verify(em).find(User.class, id, singletonMap(EntityGraphType.LOAD.getKey(), (Object) entityGraph));
 	}
+
+	@Test // DATAJPA-931
+	public void mergeGetsCalledWhenDetached() {
+
+		User detachedUser = new User();
+
+		when(em.contains(detachedUser)).thenReturn(false);
+
+		repo.save(detachedUser);
+
+		verify(em).merge(detachedUser);
+	}
+
+	@Test // DATAJPA-931
+	public void mergeGetsNotCalledWhenAttached() {
+
+		User attachedUser = new User();
+
+		when(em.contains(attachedUser)).thenReturn(true);
+
+		repo.save(attachedUser);
+
+		verify(em, never()).merge(attachedUser);
+	}
 }