如何引用仅映射到JPQL中的连接的列？

Question

Let's say I have the following two tables:

@Entity public class Foo {
   @Id private int id;
   @ManyToOne()
   @JoinColumn(name = "bar_id")
   private Bar bar;
}
@Entity public class Bar {
   @Id private int id;
   private Boolean flag;
}

And I want to write a JPQL query that changes some Bar.flag values based on a collection of Foo ids.

If this were plain SQL I'd write something like this:

UPDATE Bar SET flag = true WHERE id IN (SELECT bar_id from FOO where id = 3);

You can't translate this to JPQL however, because the bar_id column isn't mapped to a property of the entity.

As bar_id isn't directly mapped on the Foo Entity, how can I achieve this kind of query in JPQL?

Answer 1

If you want a working bulk update query, the following should work:

UPDATE Bar b SET flag = true WHERE b IN (SELECT f.bar from FOO f where id = 3);

The idea is to work with entities, when you make the subselect.

Answer 2

One of the key features of JPA is abstracting away the nitty-gritty database-details (like foreign keys) from your Java code. There are multiple ways of achieving the query you are outlining, here are a couple:

@Transactional
public void updateBarFlagForFoo(final int fooId, final boolean newFlagValue) {
    final Foo foo = em.find(Foo.class, fooId);
    foo.getBar().setFlag(newFlagValue);
}

or, in bulk:

@Transactional
public void updateFlags(final List<Integer> fooIds, final boolean newFlagValue) {
    final Query query = em.createQuery(
        "update Bar b set flag = :newFlagValue where b.id in " +
            "(select f.bar.id from Foo f where f.id in (:fooIds))");
    query.setParameter("newFlagValue", newFlagValue);
    query.setParameter("fooIds", fooIds);
    query.executeUpdate();
}

Note that such an update will not change any entity retrieved within the same transaction. Doing

final Foo foo = em.find(Foo.class, 1);
updateFlags(Arrays.asList(1), true);

will not change the actual Foo-object.

My preferred way of doing this would be something like this:

@Transactional
public void updateFlags(final List<Integer> fooIds, final boolean newFlagValue) {
    final List<Bar> bars = findBarsForFooIds(fooIds);
    for (final Bar bar : bars) {
        bar.setFlag(newFlagValue);
    }
}

private List<Bar> findBarsForFooIds(final List<Integer> fooIds) {
    final TypedQuery<Bar> q =
            em.createQuery("select distinct b from Foo f join f.bar b where f.id in (:fooIds)",
                           Bar.class);
    q.setParameter("fooIds", fooIds);
    return q.getResultList();
}   

You could of course create a @Column that maps the bar_id , but I would not recommend this, as it kind of defeats the purpose of JPA .

Answer 3

You cannot access directly the bar_id . You need to do an "extra" join between Foo and Bar to use the id information from Bar entity.

So, your JPQL query will be like this:

UPDATE Bar bar SET bar.flag = true 
WHERE bar.id IN 
  (SELECT barFoo.id from FOO foo 
    JOIN foo.bar barFoo 
    WHERE foo.id = 3);