JPA Criteria 带有 fetch 的多选

Question

我有以下型号：

@Entity
@Table(name = "SAMPLE_TABLE")
@Audited
public class SampleModel implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID")
    private Long id;

    @Column(name = "NAME", nullable = false)
    @NotEmpty
    private String name;

    @Column(name = "SHORT_NAME", nullable = true)
    private String shortName;

    @ManyToOne(fetch = FetchType.LAZY, optional = true)
    @JoinColumn(name = "MENTOR_ID")
    private User mentor;

//other fields here

//omitted getters/setters

}

现在我只想查询列：id、name、shortName和mentor引用User实体（不是完整的实体，因为它有许多其他属性，我希望获得最佳性能）。

当我写查询时：

CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<SampleModel> query = builder.createQuery(SampleModel.class);
Root<SampleModel> root = query.from(SampleModel.class);
query.select(root).distinct(true);
root.fetch(SampleModel_.mentor, JoinType.LEFT);

query.multiselect(root.get(SampleModel_.id), root.get(SampleModel_.name), root.get(SampleModel_.shortName), root.get(SampleModel_.mentor));
query.orderBy(builder.asc(root.get(SampleModel_.name)));
TypedQuery<SampleModel> allQuery = em.createQuery(query);
return allQuery.getResultList();

我有以下例外：

Caused by: org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list [FromElement{explicit,not a collection join,fetch join,fetch non-lazy properties,classAlias=generatedAlias1,role=com.sample.SampleModel.model.SampleModel.mentor,tableName=USER_,tableAlias=user1_,origin=SampleModel SampleModel0_,columns={SampleModel0_.MENTOR_ID ,className=com.sample.credential.model.User}}]
    at org.hibernate.hql.internal.ast.tree.SelectClause.initializeExplicitSelectClause(SelectClause.java:214)
    at org.hibernate.hql.internal.ast.HqlSqlWalker.useSelectClause(HqlSqlWalker.java:991)
    at org.hibernate.hql.internal.ast.HqlSqlWalker.processQuery(HqlSqlWalker.java:759)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:675)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:311)
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:259)
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:262)
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:190)
    ... 138 more

异常前查询：

SELECT DISTINCT NEW com.sample.SampleModel.model.SampleModel(generatedAlias0.id, generatedAlias0.name, generatedAlias0.shortName, generatedAlias0.mentor)
FROM com.sample.SampleModel.model.SampleModel AS generatedAlias0
LEFT JOIN FETCH generatedAlias0.mentor AS generatedAlias1
ORDER BY generatedAlias0.name ASC

我知道我可以用 join 代替 fetch 但这样我就会遇到 N+1 问题。另外，我没有从用户到 SampleModel 的反向引用，我不想有..

Answer 1

我遇到了同样的问题，并发现我可以通过使用以下方法来解决它：

CriteriaQuery<Tuple> crit = builder.createTupleQuery();

代替

CriteriaQuery<X> crit = builder.createQuery(X.class);

必须做一些额外的工作才能产生最终结果，例如在您的情况下：

return allQuery.getResultList().stream()
    map(tuple -> {
        return new SampleModel(tuple.get(0, ...), ...));
    })
    .collect(toList());