JPA Criteria API: Multiple condition on LEFT JOIN

问题:

I have a join reference like following for which the first join expression is constructed by the JPA API automatically.

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Tuple> c = cb.createTupleQuery();
Root<Demand> demands = c.from(Demand.class);
Join<Demand, Metadata> joinMetadata = demands.join("metadatas", JoinType.LEFT);

but, I would like to add an aditional condition to my joinMetadata like Metadata.type="AFFECTATION_KEY" but I don't know how.

Many thanks for any help

回答1:

JPA always joins by the mapping join columns, only. JPA 2.0 does not support an ON clause, but the 2.1 draft does have this support.

EclipseLink 2.4 has support for an ON clause,

See, http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Querying/JPQL#ON

It is also possible through the Criteria API, using the EclipseLink native Expression API that provides on clause support,

See, http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Querying/Criteria#JpaCriteriaBuilder_and_EclipseLink_Extensions

回答2:

And Hibernate's Criteria API version 3.6 supports it too:

    Criteria c = session.createCriteria(Tuple.class);

    c.createAlias("demands", "d")
    .createAlias("d.metadata", "m", 
        Criteria.LEFT_JOIN, Restrictions.eq("m.type", "AFFECTATION_KEY"));

Or you may want Restrictions.eqProperty() for that last bit. Note the under-appreciated version of createAlias() with the fourth parameter. Here is a quote from the documentation

Parameters:

• associationPath - A dot-seperated property path
• alias - The alias to assign to the joined association (for later reference).
• joinType - The type of join to use.
• withClause - The criteria to be added to the join condition (ON clause)

Hibernate API 3.6