Relax TestEntityManager @ID restriction

Change TestEntityManager so that entities with an ID can be persisted.

Fixes gh-6546

spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/orm/jpa/TestEntityManager.java

@@ -90,8 +90,6 @@ public class TestEntityManager {
 	 * @return the persisted entity
 	 */
 	public <E> E persist(E entity) {
-		Assert.state(getId(entity) == null,
-				"Entity " + entity.getClass().getName() + " already has an ID");
 		getEntityManager().persist(entity);
 		return entity;
 	}

spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/orm/jpa/TestEntityManagerTests.java

@@ -74,7 +74,7 @@ public class TestEntityManagerTests {
 	public void persistAndGetIdShouldPersistAndGetId() throws Exception {
 		bindEntityManager();
 		TestEntity entity = new TestEntity();
-		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(null, 123);
+		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(123);
 		Object result = this.testEntityManager.persistAndGetId(entity);
 		verify(this.entityManager).persist(entity);
 		assertThat(result).isEqualTo(123);
@@ -84,7 +84,7 @@ public class TestEntityManagerTests {
 	public void persistAndGetIdForTypeShouldPersistAndGetId() throws Exception {
 		bindEntityManager();
 		TestEntity entity = new TestEntity();
-		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(null, 123);
+		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(123);
 		Integer result = this.testEntityManager.persistAndGetId(entity, Integer.class);
 		verify(this.entityManager).persist(entity);
 		assertThat(result).isEqualTo(123);
@@ -99,17 +99,6 @@ public class TestEntityManagerTests {
 		assertThat(result).isSameAs(entity);
 	}
 
-	@Test
-	public void persistWhenAlreadyHasIdShouldThrowException() throws Exception {
-		bindEntityManager();
-		TestEntity entity = new TestEntity();
-		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(123);
-		this.thrown.expect(IllegalStateException.class);
-		this.thrown.expectMessage(
-				"Entity " + TestEntity.class.getName() + " already has an ID");
-		this.testEntityManager.persistAndGetId(entity, Integer.class);
-	}
-
 	@Test
 	public void persistAndFlushShouldPersistAndFlush() throws Exception {
 		bindEntityManager();
@@ -125,7 +114,7 @@ public class TestEntityManagerTests {
 		bindEntityManager();
 		TestEntity entity = new TestEntity();
 		TestEntity found = new TestEntity();
-		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(null, 123);
+		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(123);
 		given(this.entityManager.find(TestEntity.class, 123)).willReturn(found);
 		TestEntity result = this.testEntityManager.persistFlushFind(entity);
 		verify(this.entityManager).persist(entity);