persist many to many relationship with @DataJpaTest and h2 database not working

I have two entities. User:

@Entity
@Table(name = "user")
@Data
public class User {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id")
private Long id;
@Column(name = "username", unique = true)
private String username;
@ManyToMany(fetch = FetchType.EAGER)
@JoinTable(name = "user_role",
joinColumns = @JoinColumn(name = "user_id"),
inverseJoinColumns = @JoinColumn(name = "role_id"))
private Set<Role> roles = new HashSet<>();
public void addRole(Role role) {
this.roles.add(role);
}
public void addUser(User user) {
this.users.add(user);
}

Role:

@Entity
@Table(name = "role")
@Data
public class Role {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@Column(name = "name", unique = true)
private String name;
@ManyToMany(mappedBy = "roles", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
private Set<User> users = new HashSet<>();

I'm using Spring Data JPA and create repositories for User and Role:

public interface UserRepository extends JpaRepository<User, Long> {
public Optional<User> findByUsername(String username);
}

<!-- -->

public interface RoleRepository extends JpaRepository<Role, Long> {
public Optional<Role> findByName(String name);
public void deleteByName(String name);
}

I write a test which creates a new User and adds a Role to the User but Role doesn't get added to the User` and the test failes:

@ExtendWith(SpringExtension.class)
@DataJpaTest
class UserRepositoryTest {
@Autowired
UserRepository userRepository;
@Autowired
RoleRepository roleRepository;
@BeforeEach
void setUp() {
Role role1 = new Role();
role1.setName("Admin");
Role role2 = new Role();
role2.setName("Teacher");
Role role3 = new Role();
role3.setName("User");
roleRepository.saveAll(Arrays.asList(role1, role2, role3));
}
@Test
@Transactional
public void createNewUserTest() {
String username = "User1";
String password = "password";
String firstName = "name";
String lastName = "last name";
User user = new User();
user.setUsername(username);
user.setPassword(password);
user.setFirstName(firstName);
user.setLastName(lastName);
Role findedUserRole = roleRepository.findByName("User").get();
Role findedAdminRole = roleRepository.findByName("Admin").get();
user.addRole(findedUserRole);
user.addRole(findedAdminRole);
userRepository.save(user);
User findedUser = userRepository.findByUsername(username).get();
Role findedRole = roleRepository.findByName("User").get();
assertEquals(firstName,findedUser.getFirstName());
assertEquals(2, findedUser.getRoles().size());
assertTrue(findedRole.getUsers().contains(findedUser));
}
}

assertTrue(findedRole.getUsers().contains(findedUser)) fails.
If using same method in application (without adding user to role) everything is ok, but in in the test it does not work.

My application:

@Component
public class InitializeData implements CommandLineRunner {
@Autowired
private UserRepository userRepository;
@Autowired
private RoleRepository roleRepository;
@Transactional
@Override
public void run(String... args) throws Exception {
Role role1 = new Role();
role1.setName("Admin");
Role role2 = new Role();
role2.setName("Teacher");
Role role3 = new Role();
role3.setName("User");
roleRepository.saveAll(Arrays.asList(role1, role2, role3));
String username = "User1";
String password = "password";
String firstName = "name";
String lastName = "last name";
User user = new User();
user.setUsername(username);
user.setPassword(password);
user.setFirstName(firstName);
user.setLastName(lastName);
Role findedUserRole = roleRepository.findByName("User").get();
Role findedAdminRole = roleRepository.findByName("Admin").get();
user.addRole(findedUserRole);
user.addRole(findedAdminRole);
userRepository.save(user);
}

}

Everything works fine!
What is going on?

You fell in the trap of JPAs first level cache.

Your complete test happens in a single transaction and therefore session.

One of the core principles of JPA is that within a session for a given class and id the EntityManager will always return the same instance.

So when you do a Role findedRole = roleRepository.findByName("User").get(); in your test, you are not actually reloading the role. You are getting the exact instance that you created in the setup part of the test, which still has no users.

The way to fix this depends on what you actually want to achieve.

1. For bidirectional relationships you should always keep them in sync, i.e. the call to user.addRole(..) should update the Role passed as an argument. See https://vladmihalcea.com/jpa-hibernate-synchronize-bidirectional-entity-associations/ for details on that.

   While this would make your test green, it doesn't actually test if the data was written to the database and can be loaded again as expected. For this I like to use one of the following approaches.

2. flush & clear. Get the EntityManager injected in the test. After saving the entities call flush on it, in order to write the changes to the database and then clear to clear the 1st level cache. This will ensure that subsequent load operations actually hit the database and load new instances.

3. explicite transactions. Remove the @Transactional annotations from your tests. Get a TransactionTemplate injected into your test and run setup, the saving part and the loading and assertion part in separate transactions using TransactionManager.execute. I think this makes the intent of the test more obvious. But now your transactions actually commit, which might cause problems if you are reusing the same database for several tests.