mysql - JPA's Map<KEY, VALUE> query by JPQL failed

Question

I am storing a Map in JPA , which stores a keyword translation in each language . such as one object stores Locale.ENGLISH -> "Father" , Locale.CHINESE -> "PaPa". And another object stores Locale.ENGLISH -> "Mother" , Locale.CHINESE -> "MaMa";

Here is my working design :

public class Relation {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private long id;

  @ElementCollection
  @MapKeyColumn(name="locale")
  @Column(name="value")
  @CollectionTable(name = "RelationName", joinColumns = @JoinColumn(name = "relation_id"))
  private Map<Locale, String> langMap = new HashMap<>();

  // other fields skipped
}

It works well , I can store many keyword translations to DB. But when query with JPQL , it has some problems :

For example , I want to find which Relation has English key with value "Father" :

This is my code :

Relation r = em.createQuery("select r from Relation r join r.langMap m where ( KEY(m) = :locale and VALUE(m) = :value ) " , Relation.class)
  .setParameter("locale" , locale)
  .setParameter("value" , value)
  .getSingleResult();

It generates this strange / not-working SQL :

Hibernate: 
    select
        relation0_.id as id1_18_
    from
        Relation relation0_ 
    inner join
        RelationName langmap1_ 
            on relation0_.id=langmap1_.relation_id 
    where
        langmap1_.locale=? 
        and (
            select
                langmap1_.value 
            from
                RelationName langmap1_ 
            where
                relation0_.id=langmap1_.relation_id
        )=?
00:16:12.038 WARN  o.h.e.jdbc.spi.SqlExceptionHelper - SQL Error: 1242, SQLState: 21000
00:16:12.038 ERROR o.h.e.jdbc.spi.SqlExceptionHelper - Subquery returns more than 1 row

I don't know why it generates that strange subquery.

I can solve this problem by Criteria :

CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Relation> criteria = builder.createQuery(Relation.class);
Root<Relation> root = criteria.from(Relation.class);
criteria.select(root);
MapJoin<Relation , Locale , String> mapJoin = root.joinMap("langMap");
criteria.where(builder.and(
  builder.equal(mapJoin.key(), locale) ,
  builder.equal(mapJoin.value() , value))
);

return em.createQuery(criteria).getSingleResult();

It generates correct SQL ( where langmap1_.locale=? and langmap1_.value=? ) and works well.

But I feel Criteria is too complicated. And I wonder why the JPQL failed? How to correct the JPQL ?

Thanks.

Env :

JPA2 , Hibernate 4.2.3 , MySQL dialect

Answer 1

I had the same problem. It looks like accessing map by ref (without VALUE()) already gives you a map entry value, i.e. the next JPQL should be transformed to a valid SQL:

select r from Relation r join r.langMap m where ( KEY(m) = :locale and m = :value )