休眠-FetchType.LAZY和FetchType.EAGER被忽略

Question

I have an entity (named Parent) with a @OneToOne mapping to a Child entity. Currently defined with FetchType.EAGER, but it doesn't matter to the problem at hand here.

I am trying to perform a query on Parent that does a LEFT JOIN on the Child entity, rendering the FetchType setting useless (supposedly). However, the query on the Child entity still gets executed, even though the join is performed correctly.

@Entity
public class Parent {
    @NotFound(action = NotFoundAction.IGNORE)
    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    @JoinColumn(name = "id", insertable = false, updatable = false)
    private Child child;
}

@Entity
public class Child {
}

CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();
        CriteriaQuery<Parent> criteriaQuery = criteriaBuilder.createQuery(Parent.class);

Root<Parent> root = criteriaQuery.from(Parent.class);
Fetch<Parent, Child> join = root.fetch("child", JoinType.LEFT);

return session.createQuery(criteriaQuery).getSingleResult();

I would have expected this to generate a single query with a LEFT JOIN that gathers all of the data, but instead it performs:

1) SELECT * FROM parent LEFT JOIN child ...
2) SELECT * FROM child where id = ?

Answer 1

I think you should add the annotation @Fetch(FetchMode.JOIN) on the child field. JoinType just defines the type of the join, but you should tell hibernate that when loading parent, load the child (here eagerly) using join and not using extra selects.

This way you dont need to specify the join in the criteria query, you can just load parents and hibernate will load child entities using the FetchMode.JOIN

Answer 2

OK, the solution was eventually to have a Bi-Directional association. Defining both of the directions solved the issue for me - like this:

@Entity
public class Parent {
    @OneToOne(mappedBy="parent")
    @Fetch(FetchMode.JOIN)
    private Child child;
}

@Entity
public class Child {
    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "id", insertable = false, updatable = false)
    private Parent parent;
}

Thanks for the help nevertheless:)

Answer 3

Actually your criteria generates the correct query (1). But when Parent entity gets materialized, Hibernate issues the (2) query because the Child should be loaded eagerly. In other words Hibernate will always try to load the child.

There are 2 ways to escape this:

• remove EAGER fetch.
• change criteria to return a projection