Making JPA and ModelMapper work in entity updates

Question

In my crud controller, the following is happening during Update operation

Client does a request to "POST /{id}" and sends me a DTO inside request body. Then I call ModelMapper to convert the DTO into a database entity. After that, I set the Id field of entity to the one client sent me. I do a repository save.

This fails (creates a brand-new entity instead of updating) since I'm trying to set an autogenerated field by hand. JPA EntityManager has a merge method, but I'm not sure if it does what I want.

So, I change my code to: first fetch the entity with the requested id, then ask ModelMapper to overwrite its fields, and finally do a repository save. However I couldn't make ModelMapper update an existing object instead of constructing a new instance. It looks possible according the docs, but I got lost between Providers and TypeMaps and whatnot.

So what I want is to either convince JPA to do an update with a transient entity, or to convince ModelMapper to set the fields of an existing (and jpa attached) entity.

The fact that a simple task got this convoluted makes me suspect I'm doing something fundamentally wrong. Can you suggest me a solution?

Answer 1

Are you looking for skip?

public static void main(String[] args) {
    ModelMapper mapper = new ModelMapper();
    mapper.addMappings(new PropertyMap<B, A>() {
        @Override
        protected void configure() {
            skip(destination.getId());
        }
    });
    A a = new A(5, 15);
    B b = new B(7, 17);
    mapper.map(b, a);
    System.out.println(a.id); // prints 5
    System.out.println(a.x);  // prints 17
}

static class A {
    private int id;
    private long x;

    public A() {
    }

    public A(int id, long x) {
        this.id = id;
        this.x = x;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public long getX() {
        return x;
    }

    public void setX(long x) {
        this.x = x;
    }
}

static class B {
    // exact copy of A
}