why a manyToMany join table does not insert - spring-data-jpa

Question

I use all the default table name and column names, so I didn't add a jointable (actually I tried with jointable). Bidirectional, only one of the two Entities has a mappedBy.
#Entity
public class User extends AbstractPersistable<Long> {
#ManyToMany
private Set<House> houses;
#Entity
public class House extends AbstractPersistable<Long> {
#ManyToMany(mappedBy = "houses")
Set<User> users;
Should be simple enough, except that the hibernate does not insert jointable after running test:
#RunWith(SpringRunner.class)
#SpringBootTest
public class DemoSmApplicationTests {
#Test
public void contextLoads() throws IOException {
House p =new House();
Set us = new HashSet<>();
User u = userRepository.findOne(1L);
us.add(u);
p.setUsers(us);
houseRepository.save(p);
}
The console only has one "insert":
Hibernate: select user0_.id as id1_5_0_, user0_.name as name2_5_0_ from user user0_ where user0_.id=?
Hibernate: insert into house (random) values (?)

Answer 1

When we are using bidirectional associations we have to provide 'helper' methods to preserve synchronicity between both sides, for example:
public House addUsers(User... users) {
Stream.of(users).forEach(user -> {
user.getHouses().add(this);
this.users.add(user);
});
return this;
}
More other we have to set the cascade property of the #ManyToMany annotation at least to MERGE to tell Hibernate to update nested objects as well, when we update the 'parent' entity:
#ManyToMany(mappedBy = "houses", cascade = CascadeType.MERGE)
private final Set<User> users = new HashSet<>();
Then we can get the right result:
List<User> users = userRepo.save(Arrays.asList(
new User("user1"),
new User("user2"),
new User("user3")
));
List<House> houses = houseRepo.save(Arrays.asList(
new House("address1"),
new House("address2")
));
houses.get(0).addUsers(users.get(0), users.get(1));
houses.get(1).addUsers(users.get(1), users.get(2));
houseRepo.save(houses);