Fetch Lazy 不适用于具有复合 PK 的 ManyToOne

Posted 2023-04-12

【中文标题】Fetch Lazy 不适用于具有复合 PK 的 ManyToOne

【英文标题】：Fetch Lazy does not work for ManyToOne with composite PK

【发布时间】：2019-03-29 10:44:55

【问题描述】：

我有以下数据库结构

CREATE TABLE a (
    id TEXT PRIMARY KEY,
    creation_date TIMESTAMP WITH TIME ZONE NOT NULL
);

CREATE TABLE b (
    id text
    a_id TEXT REFERENCES a,
    active BOOLEAN NOT NULL,
    creation_date TIMESTAMP WITH TIME ZONE NOT NULL,
    modification_date TIMESTAMP WITH TIME ZONE NOT NULL,
    version INTEGER NOT NULL DEFAULT 0,
    PRIMARY KEY (id, a_id)
);

CREATE UNIQUE INDEX ON b (a_id) WHERE (active);
CREATE UNIQUE INDEX ON b (id) WHERE (active);

还有 JPA 实体，其中 B 对 A 有复合 PK 和 FK

@Entity
@Table (name = "a")
class A (

    @Id
    var id: String
)

@Entity
@Table (name = "b")
class B (

    @EmbeddedId
    @AttributeOverride (name = "aId", column = Column (name = "a_id", updatable = false))
    var pk: Pk,

    var active: Boolean

) 

    @JoinColumn (name = "a_id")
    @MapsId ("aId")
    @ManyToOne (cascade = [CascadeType.ALL], fetch = FetchType.LAZY)
    var a: A? = null

    @Embeddable
    class Pk (

        var id: String,

        var aId: String?
    ): Serializable 

        override fun equals (other: Any?): Boolean 
            ...
        

        override fun hashCode (): int 
            ...
        
    

存储库

interface BRepository: JpaRepository <B, B.Pk>

当我打电话时

bRepository.findById (B.Pk ("b_id", "a_id"))

然后两个请求出现在休眠日志中

Hibernate: select b0_.a_id as a_id1_1_0_, b0_.id as id2_1_0_, b0_.active as active3_1_0_, b0_.creation_date as creation4_1_0_, b0_.modification_date as modifica5_1_0, your own; and b0_.id =?
Hibernate: select a0_.id as id1_0_0_, a0_.creation_date as creation2_0_0_ from a a__ where a0_.id =?

B 实体中的 FetchType.LAZY 不起作用并加载了 A。

如何修复延迟加载的映射？

【参考方案1】：

延迟加载的要点是，当您获取 A 时，它不会获取一对多映射，因此不会获取 B。您可以使用 FetchType.EAGER 来解决这个问题。太了解 LAZY 和 EAGER Fetch 类型的区别，请看这个answer