JPA @OneToOne选择具有N + 1个查询的列表

问题描述

我实际上是在尝试使用JPA @OneToOne注释将Child实体链接到其Parent.

I'm actually trying to use JPA @OneToOne annotation to link a Child entity to its Parent.

工作正常，除了以下事实:在获取Child列表时，JPA引擎(在这种情况下为Hibernate)进行1 + n次查询.

It's working well, except the fact that when getting a list of Childs, the JPA engine (Hibernate in this case) make 1+n queries.

这是Hibernate查询的日志:

Here is the log of the Hibernate queries :

select child0_.id as id1_0_, child0_.parent as parent3_0_, child0_.value as value2_0_ from child child0_
select parent0_.id as id1_1_0_, parent0_.something as somethin2_1_0_ from parent parent0_ where parent0_.id=?
select parent0_.id as id1_1_0_, parent0_.something as somethin2_1_0_ from parent parent0_ where parent0_.id=?
select parent0_.id as id1_1_0_, parent0_.something as somethin2_1_0_ from parent parent0_ where parent0_.id=?

使用完全相同的实体定义，尤其是当我得到一个孩子时，JPA使用预期的JOIN执行查询:

Using exactly the same entities definition, when I get a child in particular, JPA executes the query with expected JOIN :

select child0_.id as id1_0_0_, child0_.parent as parent3_0_0_, child0_.value as value2_0_0_, parent1_.id as id1_1_1_, parent1_.something as somethin2_1_1_ from child child0_ left outer join parent parent1_ on child0_.parent=parent1_.id where child0_.id=?

这是Child实体定义:

@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
@Entity
@Table(name = "child")
public class Child {

    @Id
    private Long   id;
    @Column
    private String value;
    @OneToOne(optional = false)
    @JoinColumn(name = "parent")
    private Parent parent;

}

和Parent实体:

@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
@Entity
@Table(name = "parent")
public class Parent {

    @Id
    private Long   id;
    @Column
    private String something;

}

您可以在此处找到运行代码的完整示例: https://github.com/Alexandre-Carbenay/demo-jpa-onetoone

You can find a complete example of running code here :https://github.com/Alexandre-Carbenay/demo-jpa-onetoone

在使用Parent获取Child实体列表时，有没有一种方法可以避免1 + n个查询?

Is there a way to avoid the 1+n queries when getting the list of Child entities with Parent ?

推荐答案

我终于找到了一个比JOIN FETCH更好的解决方案，该解决方案也适用于QueryDsl，并在存储库方法上使用了@EntityGraph批注.

I finally found a better solution than JOIN FETCH that also works with QueryDsl, using @EntityGraph annotation on repository methods.

这是更新的Child定义:

@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
@Entity
@NamedEntityGraph(name = "Child.withParent", attributeNodes = @NamedAttributeNode("parent"))
@Table(name = "child")
public class Child {
    @Id
    private Long   id;
    @Column
    private String value;
    @OneToOne(optional = false)
    @JoinColumn(name = "parent")
    private Parent parent;
}

和ChildJpaRepository定义:

public interface ChildJpaRepository extends JpaRepository<Child, Long>, QueryDslPredicateExecutor<Child> {

    @Override
    @EntityGraph("Child.withParent")
    List<Child> findAll();

    @Override
    @EntityGraph("Child.withParent")
    List<Child> findAll(Predicate predicate);

}

感谢Simon Martinelli和Vlad Mihalcea的帮助

Thanks to Simon Martinelli and Vlad Mihalcea for your help

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