How to load only ids from Many to Many mapping tables?

Question

In a Many to Many relation between two table with a Mapping table in between, how can I only load ids for the second entity.

Following is the example to explain what

Answer 1

I encountered a problem with the accepted answer, when I wanted to map both pages and pageIds. Hibernate will use both of these fields to make changes to the mapping table when a User is saved, possibly causing a lot of weird behaviour.

I instead solved this by making pageIds @Transient and populating the collection using @PostLoad:

@ManyToMany
@JoinTable(
    name="user_page",
    joinColumns = @JoinColumn(name="id_user"),
    inverseJoinColumns = @JoinColumn(name="id_page")
)
public Set<Page> pages;

@Transient
public Set<Long> pageIds;

@PostLoad
private void postLoad() {
    pageIds = pages.stream().map(Page::getId).collect(Collectors.toSet());
}

Answer 2

In User class:

@ManyToMany
@JoinTable(
    name="user_page",
    joinColumns = @JoinColumn(name="id_user"),
    inverseJoinColumns = @JoinColumn(name="id_page")
)
public Set<Page> pages;

You can get the id's by iterating over the returned set. By default collections are lazily (i.e. only ids) loaded.

EDIT: If you don't want to map Page for some reason, you can use @ElementCollection like this:

@ElementCollection
@CollectionTable(name="user_page", joinColumns=@JoinColumn(name="id_user"))
@Column(name="id_page")
public Set<Long> pageIds;