Dedicated exception for aggregate roots without id property.

We now distinguish between an id not set during insert and a supposed aggregate root without id property.

Closes #1502
Original pull request #1855

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/JdbcAggregateTemplate.java

@@ -165,6 +165,8 @@ public class JdbcAggregateTemplate implements JdbcAggregateOperations {
 
 		Assert.notNull(instance, "Aggregate instance must not be null");
 
+		verifyIdProperty(instance);
+
 		return performSave(new EntityAndChangeCreator<>(instance, changeCreatorSelectorForSave(instance)));
 	}
 
@@ -179,6 +181,7 @@ public class JdbcAggregateTemplate implements JdbcAggregateOperations {
 
 		List<EntityAndChangeCreator<T>> entityAndChangeCreators = new ArrayList<>();
 		for (T instance : instances) {
+			verifyIdProperty(instance);
 			entityAndChangeCreators.add(new EntityAndChangeCreator<>(instance, changeCreatorSelectorForSave(instance)));
 		}
 		return performSaveAll(entityAndChangeCreators);
@@ -425,6 +428,11 @@ public class JdbcAggregateTemplate implements JdbcAggregateOperations {
 		}
 	}
 
+	private <T> void verifyIdProperty(T instance) {
+		// accessing the id property just to raise an exception in the case it does not exist.
+		context.getRequiredPersistentEntity(instance.getClass()).getRequiredIdProperty();
+	}
+
 	private <T> void doDeleteAll(Iterable<? extends T> instances, Class<T> domainType) {
 
 		BatchingAggregateChange<T, DeleteAggregateChange<T>> batchingAggregateChange = BatchingAggregateChange