我实际上是在尝试使用 JPA @OneToOne
链接 Child
的注释实体到其 Parent
.
它运行良好,除了在获取 Child
列表时s,JPA 引擎(在本例中为 Hibernate)进行 1+n 次查询。
这是 Hibernate 查询的日志:
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,特别是当我得到一个 child 时,JPA 使用预期的 JOIN 执行查询:
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
获取
Child
的列表时,有没有办法避免 1+n 查询实体 Parent
?
最佳答案
我终于找到了比 JOIN FETCH
更好的解决方案也适用于 QueryDsl,使用 @EntityGraph
关于存储库方法的注释。
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
关于performance - JPA @OneToOne 选择带有 N+1 个查询的列表,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/47754497/