org.hibernate.loader.MultipleBagFetchException：无法同时获取多个包

java hibernate jpa bag hibernate-mapping

2022-09-02 02:25:02

以下是我的代码在这里，我使用多个列表从数据库中获取数据。在从 hql 查询中获取数据时，它显示异常。

波霍级

public class BillDetails implements java.io.Serializable {

private Long billNo;
// other fields
@LazyCollection(LazyCollectionOption.FALSE)
private List<BillPaidDetails> billPaidDetailses = new ArrayList<BillPaidDetails>();
private Set productReplacements = new HashSet(0);
@LazyCollection(LazyCollectionOption.FALSE)
private List<BillProduct> billProductList = new ArrayList<BillProduct>();
//getter and setter
}

hmb.xml 文件

<class name="iland.hbm.BillDetails" table="bill_details" catalog="retail_shop">
        <id name="billNo" type="java.lang.Long">
            <column name="bill_no" />
            <generator class="identity" />
        </id>
 <bag name="billProductList" table="bill_product" inverse="true" lazy="false" fetch="join">
            <key>
                <column name="bill_no" not-null="true" />
            </key>
            <one-to-many class="iland.hbm.BillProduct" />
        </bag>
        <bag name="billPaidDetailses" table="bill_paid_details" inverse="true" lazy="false" fetch="select">
            <key>
                <column name="bill_no" not-null="true" />
            </key>
            <one-to-many class="iland.hbm.BillPaidDetails" />
        </bag>
        <set name="productReplacements" table="product_replacement" inverse="true" lazy="false" fetch="join">
            <key>
                <column name="bill_no" not-null="true" />
            </key>
            <one-to-many class="iland.hbm.ProductReplacement" />
        </set>
    </class>

Hql 查询

String hql = "select distinct bd,sum(bpds.amount) from BillDetails as bd "
                    + "left join fetch bd.customerDetails as cd "
                    + "left join fetch bd.billProductList as bpd "
                    + "left join fetch bpd.product as pd "
                    +"left join fetch bd.billPaidDetailses as bpds "
                    + "where bd.billNo=:id "
                    + "and bd.client.id=:cid ";

我正在尝试以下查询以从数据库获取数据，但这显示了如何解决此问题org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags

答案 1

Hibernate不允许获取多个袋子，因为这会产生笛卡尔积。

现在，您会发现许多答案，博客文章，视频或其他资源，告诉您对集合使用a而不是a。SetList

这是一个可怕的建议！

使用而不是会使消失，但笛卡尔积仍然存在。SetsListsMultipleBagFetchException

正确的修复

而不是在单个 JPQL 或 Criteria API 查询中使用多个：JOIN FETCH

List<Post> posts = entityManager.createQuery("""
    select p
    from Post p
    left join fetch p.comments
    left join fetch p.tags
    where p.id between :minId and :maxId
    """, Post.class)
.setParameter("minId", 1L)
.setParameter("maxId", 50L)
.getResultList();

您可以执行以下技巧：

List<Post> posts = entityManager.createQuery("""
    select distinct p
    from Post p
    left join fetch p.comments
    where p.id between :minId and :maxId
    """, Post.class)
.setParameter("minId", 1L)
.setParameter("maxId", 50L)
.setHint(QueryHints.PASS_DISTINCT_THROUGH, false)
.getResultList();

posts = entityManager.createQuery("""
    select distinct p
    from Post p
    left join fetch p.tags t
    where p in :posts
    """, Post.class)
.setParameter("posts", posts)
.setHint(QueryHints.PASS_DISTINCT_THROUGH, false)
.getResultList();

只要您最多使用获取一个集合，就可以了。通过使用多个查询，您将避免笛卡尔积，因为任何其他集合，但第一个集合是使用辅助查询获取的。JOIN FETCH

答案 2

对我来说，我有同样的错误，我通过添加休眠的注释来解决@Fetch

@OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
@Fetch(value = FetchMode.SUBSELECT)
private List<Child> childs;