Using the JPA Criteria API, can you do a fetch join that results in only one join?

Question

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 re

Answer 1

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);

Answer 2

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");

Answer 3

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");
...