java - Hibernate 在获取实体集合后查询实体

Question

我们有以下2个类

public class StagingConcept implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO, generator = "sequence")
    @SequenceGenerator(name = "sequence", sequenceName = "stg_concept_seq")
    @Column(name = "id")
    private long id;

    @Column(name = "concept_id", nullable = false, length = 18)
    private String conceptId;

    @OneToMany(mappedBy = "concept", fetch = FetchType.LAZY, 
                cascade = { CascadeType.ALL })
    private Set<StagingConceptDescription> descriptions;

    // rest of the class
}

public class StagingConceptDescription {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO, generator = "sequence")
    @SequenceGenerator(name = "sequence", sequenceName = "stg_concept_desc_seq")
    @Column(name = "id")
    private long id;

    @ManyToOne
    @JoinColumn(name = "concept_id", referencedColumnName = "concept_id")
    @ForeignKey(name = "stg_concept_desc_fk1")
    private StagingConcept concept;
    // rest of the class
}

为了保持示例的精确性，一些细节(例如其他类属性和实体注释)已被省略。如果您需要更多详细信息，请告诉我。是的，从 StagingConceptDescription 到 StagingConcept 的 FK 是非 PK 外键。

当我创建标准时:

"from " + StagingConcept.class.getCanonicalName()

我通过一个查询从数据库获取所有 StagingConcept 实体。但我需要获取每个 StagingConcept 的描述。为此，我编写了一个查询:

"from " + StagingConcept.class.getCanonicalName() + " join fetch descriptions"

生成的 SQL 如下所示:

select stagingcon0_.id as col_0_0_,
descriptio1_.id as id178_1_,
stagingcon0_.is_active as is2_149_0_,
stagingcon0_.brand_restriction_status as brand3_149_0_,
stagingcon0_.concept_id as concept4_149_0_,
stagingcon0_.container_type as container5_149_0_,
stagingcon0_.controlled_drug_status as controlled6_149_0_,
stagingcon0_.effective_date as effective7_149_0_,
stagingcon0_.form as form149_0_,
stagingcon0_.is_multi_component as is9_149_0_,
stagingcon0_.namespace as namespace149_0_,
stagingcon0_.preferred_term as preferred11_149_0_,
stagingcon0_.source as source149_0_,
stagingcon0_.source_version as source13_149_0_,
stagingcon0_.subsidy_status as subsidy14_149_0_,
stagingcon0_.type as type149_0_,
stagingcon0_.unit_of_use_size as unit16_149_0_,
stagingcon0_.unit_of_use_size_unit as unit17_149_0_,
descriptio1_.is_active as is2_178_1_,
descriptio1_.concept_id as concept6_178_1_,
descriptio1_.is_preferred as is3_178_1_,
descriptio1_.term as term178_1_,
descriptio1_.type as type178_1_,
descriptio1_.concept_id as concept6_149_0__,
descriptio1_.id as id0__
from stg_concept stagingcon0_ 
inner join stg_concept_description descriptio1_ on stagingcon0_.concept_id=descriptio1_.concept_id

它确实获取所有 StagingConcepts 及其描述，尽管该 SQL 中的结果集稍大一些。

到目前为止一切看起来都很好。但随后它又尝试为每一个描述找到一个分阶段概念。因此，如果我有 30000 个分期概念和 60000 个描述，它将发送另外 60000 个查询来获取每个描述的分期概念。这看起来有点令人讨厌，并且占用了大量的时间，足以超过事务超时。

为了尝试解决此问题，我将 StagingConceptDescription 更改为

public class StagingConceptDescription {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO, generator = "sequence")
    @SequenceGenerator(name = "sequence", sequenceName = "stg_concept_desc_seq")
    @Column(name = "id")
    private long id;

    @ManyToOne(fetch = FetchType.LAZY, optional=false)
    @JoinColumn(name = "concept_id", referencedColumnName = "concept_id")
    @ForeignKey(name = "stg_concept_desc_fk1")
    private StagingConcept concept;
    // rest of the class
}

因此，ManyToOne 关系现在显式设置为 LAZY。而且，该关系声明该概念不是可选的，试图表明该关系是非可选的。通过设置，我的意思是告诉 hibernate 如果需要的话，创建一个代理对象应该没问题，因为关系的另一端总是存在的。但这些都没有任何效果。不太确定这是否有效。

我还尝试了@Fetch注释等，但都不起作用。即使将其设置为 @LazyToOne(LazyToOneOption.PROXY) 也没有任何效果。

Answer 1

基于https://stackoverflow.com/a/29863982/5464931 ，您可以进行另一次连接获取。

例如

"from " + StagingConcept.class.getCanonicalName() + " join fetch descriptions join fetch descriptions.concept"

但这不是最好的解决方案，因为它仍然再次查询实体/父/概念，这是不必要的。