Link should not be created for null references [DATAREST-1415]

ecattez opened DATAREST-1415 and commented

Hi !

I have a ManyToOne (OneToOne leads to the same issue) between Library and Book. In fact, in my case, I would like a book to be in one and only one library.

Library.java

@OneToMany
public Set<Book> getBooks() {
    return books;
}

Book.java

@ManyToOne
public Library getLibrary() {
    return library;
}

I allow the creation of a book, even if a library does not exist yet. So basically :

Book#getLibrary() is null

The problem is that LinkCollector / LinkCollectingAssociationHandler do not check the value. It results in the creation of a link to an unknown library that leads to a 404 Not Found :

{
    "name": "Discworld",
    "author": "Terry Pratchett",
    "_links": {
        "self": {
            "href": "http://localhost:8080/books/9783492285100"
        },
        "book": {
            "href": "http://localhost:8080/books/9783492285100"
        },
        "library": {
            "href": "http://localhost:8080/books/9783492285100/library"
        }
    }
}

The link association should not be created in such a case because it does not exist yet. The library link should not be present until the association of the two entities really happens

Affects: 3.1.9 (Lovelace SR9)