Spring Data @OneToMany(fetch=FetchType.EAGER) 用作选择查询模式

Question

我附在我的代码下方。当我得到所有父实体时，一个一个地加载孩子。如何解决？

父实体类。它具有 3 个属性，即 id、name 和 children。孩子属性加载急切。因为我需要父对象及其子对象。

    @Entity
    @Table(name = "parent")
    public class Parent {
        @Id
        private int id;

        private String name;

        @OneToMany(cascade = CascadeType.ALL, mappedBy = "parent", fetch = FetchType.EAGER, orphanRemoval = true)
        private List<Child> children;

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public List<Child> getChildren() {
            return children;
        }

        public void setChildren(List<Child> children) {
            this.children = children;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }

儿童班。它具有 3 个属性，例如 id、name 和 parent。父属性加载为惰性模式。因为我从不想要子对象和它的父对象。

    @Entity
    @Table(name = "child")
    public class Child {
        @Id
        private int id;

        @Column(name = "name")
        private String name;

        @JsonIgnore
        @ManyToOne(fetch = FetchType.LAZY)
        @JoinColumn(name = "parent", referencedColumnName = "id")
        private Parent parent;

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public Parent getParent() {
            return parent;
        }

        public void setParent(Parent parent) {
            this.parent = parent;
        }
    }

我尝试让所有父母使用以下代码。

    protected List getAll(){
            EntityManager entityManager = entityManagerFactory.createEntityManager();
            entityManager.getTransaction().begin();
            List<Parent> objects = entityManager.createQuery("Select o from Parent o", Parent.class).getResultList();
            entityManager.getTransaction().commit();
            entityManager.close();
            return objects;
        }

我希望只有一个连接查询。但是hibernate调用会一一查询。它得到 n+1 查询问题。

    Hibernate: select parent0_.id as id1_8_, parent0_.name as name2_8_ from parent parent0_
    Hibernate: select children0_.parent as parent3_0_0_, children0_.id as id1_0_0_, children0_.id as id1_0_1_, children0_.name as name2_0_1_, children0_.parent as parent3_0_1_ from child children0_ where children0_.parent=?
    Hibernate: select children0_.parent as parent3_0_0_, children0_.id as id1_0_0_, children0_.id as id1_0_1_, children0_.name as name2_0_1_, children0_.parent as parent3_0_1_ from child children0_ where children0_.parent=?
    Hibernate: select children0_.parent as parent3_0_0_, children0_.id as id1_0_0_, children0_.id as id1_0_1_, children0_.name as name2_0_1_, children0_.parent as parent3_0_1_ from child children0_ where children0_.parent=?

Answer 1

JPA 没有提供任何关于映射注解以选择获取策略的规范。

使用JOIN FETCH 在一个查询中获取根实体及其映射的实体/集合，从而解决了 N+1 查询问题。喜欢

SELECT distinct p FROM Parent p LEFT JOIN FETCH p.children