我有以下实体，一个 item 最多可以有两个类别，一个主要的和一个次要的。 这两个类别都使用 JoinColumnsOrFormulas 将 ManyToOne 映射到 category 表。 第一个按预期获取 EAGER，但第二个未出现在 SQL 语句中并延迟加载。 这种延迟加载会导致经典的 n+1 问题。

这是我的项目实体，其中包含应该加入的两个类别实体:

@Entity
@Table(name = "item", schema = "public", catalog = "stackoverflow_question")
@DynamicUpdate
public class Item extends StackOverflowQuestionEntity {

    @Id
    @Column(name = "id")
    protected Long id;

    @Column(name = "site")
    private String site;

    @ManyToOne
    @JoinColumnsOrFormulas({
            @JoinColumnOrFormula(formula = @JoinFormula(value = "site", referencedColumnName = "site")),
            @JoinColumnOrFormula(formula = @JoinFormula(value = "primary_category_id", referencedColumnName = "category_id"))
    })
    private Category primaryCategory;

    @Column(name = "primary_category_id")
    private Long primaryCategoryId;

    @ManyToOne
    @JoinColumnsOrFormulas({
            @JoinColumnOrFormula(formula = @JoinFormula(value = "site", referencedColumnName = "site")),
            @JoinColumnOrFormula(formula = @JoinFormula(value = "secondary_category_id", referencedColumnName = "category_id"))
    })
    private Category secondaryCategory;

    @Column(name = "secondary_category_id")
    private Long secondaryCategoryId;
}

这是类别实体:

@Entity
@Table(name = "category", schema = "public", catalog = "stackoverflow_question")
public class Category extends StackOverflowQuestionEntity {

    @Column(name = "category_id")
    private Long categoryId;

    @Column(name = "name")
    private String name;

    @Column(name = "site")
    private String site;
}

生成的查询仅包含主要类别:

SELECT this_.id AS id1_9_9_,
       this_.inserted AS inserted2_9_9_,
       this_.updated AS updated3_9_9_,
       this_.primary_category_id AS formula174_9_,
       this_.secondary_category_id AS formula176_9_,
       category2_.id AS id1_0_0_,
       category2_.inserted AS inserted2_0_0_,
       category2_.updated AS updated3_0_0_,
       category2_.name AS name7_0_0_
FROM public.item this_
LEFT OUTER JOIN public.category category2_ ON this_.site=category2_.site
AND this_.primary_category_id=category2_.category_id
WHERE True;

因此次要类别懒惰加入:

SELECT category0_.id AS id1_0_0_,
       category0_.inserted AS inserted2_0_0_,
       category0_.updated AS updated3_0_0_,
       category0_.name AS name4_0_0_,
       category0_.site AS site5_0_0_
FROM public.category category0_
WHERE category0_.site=?
  AND category0_.category_id=?;

为什么Hibernate加入secondary category lazy，注解好像是一样的

我使用的 hibernate 版本是 5.0.10.Final。

这是基本实体的样子:

@MappedSuperclass
abstract public class StackOverflowQuestionEntity implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", unique = true, insertable = true, updatable = false, nullable = false)
    protected Long id;

    @Type(type="LocalDateTime")
    @Column(name = "created", nullable = false, insertable = true, updatable = false)
    protected LocalDateTime created;

    @Type(type="LocalDateTime")
    @Column(name = "refreshed", nullable = false, insertable = true, updatable = true)
    protected LocalDateTime refreshed;

    @PreUpdate
    protected void onUpdate() {
        refreshed = now();
    }

    @PrePersist
    protected void onCreate() { 
        created = refreshed = now();
    }
}

这是一个“查询”示例，正如我所说，我正在使用 hibernate 条件和 HQL，这两种方法都会出现问题。

session
    .createCriteria(Item.class)
    .add(eq("id", id))
    .uniqueResult();

最佳答案

使用标准 JPA 注释，它看起来像这样(已更新):

@ManyToOne
@JoinColumns({
    @JoinColumn(name="site", referencedColumnName="site", insertable = false, updatable = false),
    @JoinColumn(name="primary_category_id", referencedColumnName="category_id", insertable = false, updatable = false)
})
private Category primaryCategory;

@ManyToOne
@JoinColumns({
    @JoinColumn(name="site", referencedColumnName="site", insertable = false, updatable = false),
    @JoinColumn(name="secondary_category_id", referencedColumnName="category_id", insertable = false, updatable = false)
})
private Category secondaryCategory;

更新: 我发现第二个 select 语句仅在您通过复合键使用 join 时生成:Hibernate 尝试解析关联对于 {site=site, id=null} 使用 TwoPhaseLoad。但是如果你写

@ManyToOne
@JoinColumn(name="secondary_category_id")
private Category secondaryCategory;

和secondary_category_id为null则生成唯一的select语句，secondaryCategory值将为 null。也许它会以某种方式帮助你。例如，您可以在构建条件时在 site 字段上添加约束:

Category c = (Category) session.createCriteria(Category.class)
    .add(Restrictions.eq("id", 1L)) // for example
    // here you define additional restriction on site field
    .createAlias("secondaryCategory", "sc", JoinType.LEFT_OUTER_JOIN, Restrictions.sqlRestriction("this_.site = {alias}.site"))
    .uniqueResult();

关于java - 在一个表上 hibernate 两个 ManyToOne 关系，第一个获取 Eager，第二个加载 LAZY，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/47656780/