Dealing with m-to-n relations in #RepositoryRestResource - spring

Question

Preface
I want to create a sub-resource of another resource in one call. These resources have a #ManyToMany relationship: Users and Groups.
I do not want to create first a user, then the group and after that the relation as it is shown in Working with Relationships in Spring Data REST - simply because I think a resource that cannot exist on its own, such as a group, should only be created if at least one user is also associated with that resource. For this I require a single endpoint like this one (which is not working for me, otherwise I wouldn't be here) that creates a group and also sets the associated "seeding" user in one transaction.
Currently, the only way to make this work for me is to "synchronize" the relation manually like this:
public void setUsers(Set<AppUser> users) {
users.forEach(u -> u.getGroups().add(this));
this.users = users;
}
this would allow me to
POST http://localhost:8080/groups
{
"name": "Group X",
"users": ["http://localhost:8080/users/1"]
}
but my problem with that is that this does not feel right to me - it does seem like a workaround and not the actual Spring-way to make this requirement work. So ..
I'm currently struggling with creating relational resources using Spring's #RepositoryRestResource. I want to create a new group and associate it with the calling user like this:
POST http://localhost:8080/users/1/groups
{
"name": "Group X"
}
but the only result is the response 204 No Content. I have no idea why. This may or may not be related to another question of mine (see here) where I try to achieve the same by setting the relating resource in the JSON payload - that doesn't work either.
Server side I am getting the following error:
tion$ResourceSupportHttpMessageConverter : Failed to evaluate Jackson deserialization for type [[simple type, class org.springframework.hateoas.Resources<java.lang.Object>]]: java.lang.NullPointerException
Please let me know in case you need any specific code.
Tried
I added exported = false to the #RepositoryRestResource of UserGroupRepository:
#RepositoryRestResource(collectionResourceRel = "groups", path = "groups", exported = false)
public interface UserGroupRepository extends JpaRepository<UserGroup, Long> {
List<UserGroup> findByName(#Param("name") String name);
}
and I am sending:
PATCH http://localhost:8080/users/1
{
"groups": [
{
"name": "Group X"
}
]
}
However, the result is still just 204 No Content and a ResourceNotFoundException on the server side.
Unit Test
Essentially, the following unit test is supposed to work but I can also live with an answer why this cannot work and which also shows how this is done correctly.
#Autowired
private TestRestTemplate template;
private static String USERS_ENDPOINT = "http://localhost:8080/users/";
private static String GROUPS_ENDPOINT = "http://localhost:8080/groups/";
// ..
#Test
#DirtiesContext(classMode = ClassMode.BEFORE_EACH_TEST_METHOD)
public void whenCreateUserGroup() {
// Creates a user
whenCreateAppUser();
ResponseEntity<AppUser> appUserResponse = template.getForEntity(USERS_ENDPOINT + "1/", AppUser.class);
AppUser appUser = appUserResponse.getBody();
UserGroup userGroup = new UserGroup();
userGroup.setName("Test Group");
userGroup.setUsers(Collections.singleton(appUser));
template.postForEntity(GROUPS_ENDPOINT, userGroup, UserGroup.class);
ResponseEntity<UserGroup> userGroupResponse = template.getForEntity(GROUPS_ENDPOINT + "2/", UserGroup.class);
Predicate<String> username = other -> appUser.getUsername().equals(other);
assertNotNull("Response must not be null.", userGroupResponse.getBody());
assertTrue("User was not associated with the group he created.",
userGroupResponse.getBody().getUsers().stream()
.map(AppUser::getUsername).anyMatch(username));
}
However, the line
userGroup.setUsers(Collections.singleton(appUser));
will break this test and return a 404 Bad Request.

Answer 1

According to SDR reference:
POST
Only supported for collection associations. Adds a new element to the collection. Supported media types:
text/uri-list - URIs pointing to the resource to add to the association.
So to add group to user try to do this:
POST http://localhost:8080/users/1/groups (with Content-Type:text/uri-list)
http://localhost:8080/groups/1
Additional info.