如何通过JPA Entity Graph结合JPQL/QDSL实现左连接单查询加载关联实体

我定义了如下AcquiringOperation实体，关联的CardData采用FetchType.LAZY加载策略，并配置了NamedEntityGraph：

@Entity
@Table(name = "ACQ_OPERATION")
@Getter
@Setter
@NamedEntityGraph(
        name = "acquiringoperation-entity-graph",
        attributeNodes = {
                @NamedAttributeNode("cardData"),
        }
)
public class AcquiringOperation extends GenericEntity<Long> {
    ...

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "card_data_id")
    private CardData cardData;

    ...
}

我希望通过左连接单查询同时获取AcquiringOperation和关联的CardData，但使用JPQL结合Entity Graph时出现了N+1查询问题：

EntityGraph<AcquiringOperation> entityGraph = (EntityGraph<AcquiringOperation>) dataService.getEntityManager().getEntityGraph("acquiringoperation-entity-graph");

acquiringOperations = dataService.getEntityManager().createQuery("select p from AcquiringOperation p", AcquiringOperation.class)
        .setHint("jakarta.persistence.fetchgraph", entityGraph)
        .getResultList().stream().limit(10).collect(toList());

这段代码会先查询所有AcquiringOperation，再为每条记录单独查询CardData。但使用Spring Data Repository的@EntityGraph注解时却能生成左连接单查询：

public interface AcquiringOperationRepository extends BaseRepository<AcquiringOperation> {
    @Override
    @EntityGraph(attributePaths = {"cardData"})
    List<AcquiringOperation> findAll();
}

acquiringOperations = acquiringOperationRepository.findAll().stream().limit(10).collect(toList());

请问如何通过JPQL或QDSL结合JPA Entity Graph实现这种左连接单查询？

1. 切换使用loadgraph提示而非fetchgraph #

你之前用的jakarta.persistence.fetchgraph会严格只加载Entity Graph中指定的属性，对于ManyToOne这类关联，JPA规范允许提供者选择延迟加载（即使配置了Graph）。而Spring Data的@EntityGraph默认使用loadgraph策略，它会加载Entity Graph指定的属性以及实体的其他非延迟属性，并且会主动通过连接加载关联属性。

修改JPQL代码，替换提示为jakarta.persistence.loadgraph：

EntityGraph<AcquiringOperation> entityGraph = (EntityGraph<AcquiringOperation>) dataService.getEntityManager().getEntityGraph("acquiringoperation-entity-graph");

acquiringOperations = dataService.getEntityManager().createQuery("select p from AcquiringOperation p", AcquiringOperation.class)
        .setHint("jakarta.persistence.loadgraph", entityGraph) // 替换为loadgraph
        .getResultList().stream().limit(10).collect(toList());

2. JPQL中显式使用LEFT JOIN FETCH #

如果想直接控制查询逻辑，可以在JPQL里显式编写左连接抓取语句，无需依赖Entity Graph即可实现单查询：

acquiringOperations = dataService.getEntityManager().createQuery(
        "select p from AcquiringOperation p LEFT JOIN FETCH p.cardData", 
        AcquiringOperation.class
).getResultList().stream().limit(10).collect(toList());

该方式会直接生成包含左连接的SQL，一次性加载所有所需数据，彻底避免N+1问题。

3. QDSL结合Entity Graph的实现 #

使用QDSL时，可以通过JPAQuery设置Entity Graph提示，同样推荐用loadgraph：

QAcquiringOperation acquiringOperation = QAcquiringOperation.acquiringOperation;
EntityGraph<AcquiringOperation> entityGraph = (EntityGraph<AcquiringOperation>) dataService.getEntityManager().getEntityGraph("acquiringoperation-entity-graph");

List<AcquiringOperation> acquiringOperations = new JPAQuery<>(dataService.getEntityManager())
        .select(acquiringOperation)
        .from(acquiringOperation)
        .setHint("jakarta.persistence.loadgraph", entityGraph)
        .fetch()
        .stream().limit(10).collect(Collectors.toList());

也可以在QDSL中显式添加左连接抓取：

List<AcquiringOperation> acquiringOperations = new JPAQuery<>(dataService.getEntityManager())
        .select(acquiringOperation)
        .from(acquiringOperation)
        .leftJoin(acquiringOperation.cardData).fetchJoin()
        .fetch()
        .stream().limit(10).collect(Collectors.toList());

关键原因说明 #

• fetchgraph：严格仅加载Entity Graph列出的属性，对于标记为LAZY的关联，JPA提供者可能仍选择延迟加载，导致后续N+1查询。
• loadgraph：加载Entity Graph指定属性的同时，保留实体默认加载规则，且会主动抓取Graph中的关联，这也是Spring Data @EntityGraph的默认行为，因此能生成左连接单查询。

内容的提问来源于stack exchange，提问作者Alex Zhulin