使用JPA CRUD存储库查找子实体的计数

// 父实体类
@Entity
public class ParentEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @OneToMany(mappedBy = "parent")
    private List<ChildEntity> children;

    // 省略其他属性和方法
}

// 子实体类
@Entity
public class ChildEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "parent_id")
    private ParentEntity parent;

    // 省略其他属性和方法
}

// JPA CRUD存储库接口
public interface ParentEntityRepository extends JpaRepository<ParentEntity, Long> {
    @Query("SELECT COUNT(c) FROM ParentEntity p JOIN p.children c WHERE p.id = :parentId")
    Long countChildrenByParentId(@Param("parentId") Long parentId);
}

// 调用查询方法
Long childCount = parentEntityRepository.countChildrenByParentId(parentId);