Using the JPA Criteria API, can you do a fetch joi

问题:

Using JPA 2.0. It seems that by default (no explicit fetch), @OneToOne(fetch = FetchType.EAGER) fields are fetched in 1 + N queries, where N is the number of results containing an Entity that defines the relationship to a distinct related entity. Using the Criteria API, I might try to avoid that as follows:

CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<MyEntity> query = builder.createQuery(MyEntity.class);
Root<MyEntity> root = query.from(MyEntity.class);
Join<MyEntity, RelatedEntity> join = root.join("relatedEntity");
root.fetch("relatedEntity");
query.select(root).where(builder.equals(join.get("id"), 3));

The above should ideally be equivalent to the following:

SELECT m FROM MyEntity m JOIN FETCH myEntity.relatedEntity r WHERE r.id = 3

However, the criteria query results in the root table needlessly being joined to the related entity table twice; once for the fetch, and once for the where predicate. The resulting SQL looks something like this:

SELECT myentity.id, myentity.attribute, relatedentity2.id, relatedentity2.attribute 
FROM my_entity myentity 
INNER JOIN related_entity relatedentity1 ON myentity.related_id = relatedentity1.id 
INNER JOIN related_entity relatedentity2 ON myentity.related_id = relatedentity2.id 
WHERE relatedentity1.id = 3

Alas, if I only do the fetch, then I don't have an expression to use in the where clause.

Am I missing something, or is this a limitation of the Criteria API? If it's the latter, is this being remedied in JPA 2.1 or are there any vendor-specific enhancements?

Otherwise, it seems better to just give up compile-time type checking (I realize my example doesn't use the metamodel) and use dynamic JPQL TypedQueries.

回答1:

Instead of root.join(...) you can use root.fetch(...) which returns Fetch<> object.

Fetch<> is descendant of Join<> but it can be used in similar manner.

You just need to cast Fetch<> to Join<> it should work for EclipseLink and Hibernate

...
Join<MyEntity, RelatedEntity> join = (Join<MyEntity, RelatedEntity>)root.fetch("relatedEntity");
...

回答2:

Starting with JPA 2.1 you can use dynamic entity graphs for this. Remove your fetch and specify an entity graph as follows:

CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<MyEntity> query = builder.createQuery(MyEntity.class);
Root<MyEntity> root = query.from(MyEntity.class);
Join<MyEntity, RelatedEntity> join = root.join("relatedEntity");
query.select(root).where(builder.equal(join.get("id"), 3));
EntityGraph<MyEntity> fetchGraph = entityManager.createEntityGraph(MyEntity.class);
fetchGraph.addSubgraph("relatedEntity");
entityManager.createQuery(query).setHint("javax.persistence.loadgraph", fetchGraph);

回答3:

Using root.fetch() on EclipseLink will create a SQL with INNER JOIN because have 3 types and the default is INNER.

INNER,  LEFT,  RIGHT;

The suggestion is to use CreateQuery.

TypedQuery<T> typedQuery = entityManager.createQuery(query);

EDIT: You can Cast the root to From like this:

From<?, ?> join = (From<?, ?>) root.fetch("relatedEntity");