Question

想象一下这个数据模型：

@Entity
@Data
public class A {

    @Id
    long id;

    @ManyToOne
    @JoinColumn
    @NotFound(action = NotFoundAction.IGNORE)
    B b;
}

@Entity
@Data
public class B {

    @Id
    long id;
}

现在想象以下数据（是的，它在数据库中不一致）：

A
  id | b_id
  ---------
  1  | 1
  2  | 2

B
  id
  ---
  1

如您所见，A＃2的FK到B＃2实际上是缺失的。

这使得Hibernate执行额外的SELECT请求以检查它是否实际丢失，尽管它已经在发出SELECT A LEFT JOIN B请求时发现了该请求。是否有可能以某种方式阻止执行此附加请求？

当我拿A＃1时会发生什么：

2018-01-17 18:43:05.545 DEBUG 5332 --- [           main] org.hibernate.SQL                        : select a0_."id" as id1_0_0_, a0_."b_id" as b_id2_0_0_, b1_."id" as id1_1_1_ from TEST.A a0_ left outer join TEST.B b1_ on a0_."b_id"=b1_."id" where a0_."id"=?
2018-01-17 18:43:05.555 TRACE 5332 --- [           main] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [BIGINT] - [1]
2018-01-17 18:43:05.561 TRACE 5332 --- [           main] o.h.type.descriptor.sql.BasicExtractor   : extracted value ([id1_1_1_] : [BIGINT]) - [1]
2018-01-17 18:43:05.565 TRACE 5332 --- [           main] o.h.type.descriptor.sql.BasicExtractor   : extracted value ([b_id2_0_0_] : [BIGINT]) - [1]

对于A＃2而言：

2018-01-17 18:42:13.597 DEBUG 5283 --- [           main] org.hibernate.SQL                        : select a0_."id" as id1_0_0_, a0_."b_id" as b_id2_0_0_, b1_."id" as id1_1_1_ from TEST.A a0_ left outer join TEST.B b1_ on a0_."b_id"=b1_."id" where a0_."id"=?
2018-01-17 18:42:13.605 TRACE 5283 --- [           main] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [BIGINT] - [2]
2018-01-17 18:42:13.611 TRACE 5283 --- [           main] o.h.type.descriptor.sql.BasicExtractor   : extracted value ([id1_1_1_] : [BIGINT]) - [null]
2018-01-17 18:42:13.615 TRACE 5283 --- [           main] o.h.type.descriptor.sql.BasicExtractor   : extracted value ([b_id2_0_0_] : [BIGINT]) - [2]
2018-01-17 18:42:13.617 DEBUG 5283 --- [           main] org.hibernate.SQL                        : select b0_."id" as id1_1_0_ from TEST.B b0_ where b0_."id"=?
2018-01-17 18:42:13.617 TRACE 5283 --- [           main] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [BIGINT] - [2]

Answer 1

您可以在查询中尝试JOIN FETCH，而不是简单的JOIN。 @ManyToOne关联的默认行为是急切的，除非您将其指定为LAZY，因此我不知道为什么会发生这种情况。无论如何，获取连接应解决它。

Hibernate为每个缺少的FK

1 个答案: