ManyToMany Correct implementation

Hibernate ORM
1

Hi All,
I’m using Hibernate a lot in my new project and combine it with QueryDSL which I started implementing instead of native SQL queries which became cumbersome and hard to maintain.
The biggest knowledge gap I have is how to properly map relations in my project, ManyToMany in particular.
I’ve read the article Vlad posted (thank you for your hard work it is a lifesaver), about proper handling this relational type.
However, I have a helper class which I use in my integration test which constructs entities at the repository level (so I can test all layers of my code) and when I use the above add method I get an NPE exception which indicates that the initialization of my classes property (one side of the relation) wasn’t initialized. Here is the code block:

private Entity create(Long accountId, String name, Set<AnotherEntity> others) {
        var entity = Entity.builder()
                .accountId(accountId)
                .name(name)
                .build();
        repository.save(entity);
        other.forEach(entity::addOther); -> NPE
        return entity;
    }

Here is how the relations are organized:

     /* Inside Entity class */
    @ManyToMany(cascade = {
            CascadeType.PERSIST,
            CascadeType.MERGE
    })
    @JoinTable(name = "entity_other",
            joinColumns = @JoinColumn(name = "entity_id"),
            inverseJoinColumns = @JoinColumn(name = "other_id")
    )
    private Set<OtherEntity> others = new HashSet<>();

    public void addOther(OtherEntity other) {
        others.add(other); // NPE like others wasn't initialized to new HashSet<>()
        other.getEntities().add(this);
    }

   /* Inside Other class */
    @ToString.Exclude
    @ManyToMany(mappedBy = "others")
    private Set<Entity> entities = new HashSet<>();

I’ve just changed names for the purpose of hiding the business logic.
I’m wondering do we need to implement manipulations with the repository in a thin layer that we can call the Service layer or OtherCrud.class which should provide us access to the repository and escape usage of the repository outside of the above-mentioned service class in general.
Also if there are some useful things to note about the relationship please let me know.
Thank you very much in advance.

2

Hi,
So the issue was in using the Builder by Lombok which generates a null value upon initialization.
Does this mean that when providing a “creator” for e.g. Other you should not have a setter for that entity since it can only be added through Entity class and the add method?

3

you should use something like if you are using lombok

@Builder.Default
private Set others = new HashSet<>();

4

Hi,
I managed to make this work and relation is saved from parent side of the relationship.
However when I try to use the Repository of the child (child was modified so it still has eager loading ManyToOne) to see does the child contain parent it doesn’t persist the parent.

I can’t wrap my head around this, can you guys help?