使用 JPQL 或 HQL 时如何将 @OneToMany 字段映射到 List<DTO>？

Question

我有以下实体：

@Entity
public class CityExpert {
    @Id
    private long id;

    @OneToOne
    private User user;

    @OneToMany(mappedBy = "cityExpert")
    private List<CityExpertDocument> documents;

    // Lots of other fields...
}

@Entity
public class CityExpertDocument {

    @Id
    private long id;

    @ManyToOne
    private CityExpert cityExpert;

    // Lots of other fields...
}

@Entity
public class User {
    @Id
    private long id;

    private String name;

    private String email;

    // Lots of other fields...
}

我有以下 HQL 查询，其中我选择了 CityExperts 的子集：

"select " +
        "e " +
"from " +
        "CityExpert e " +
"where " +
        "( (lower(e.user.name) like concat('%', lower(?1), '%') or e.user.name is null) or ?1 = '' ) " +
        "and " +
        "( (lower(e.user.phone) like concat('%', lower(?2), '%') or e.user.phone is null) or ?2 = '' ) "

但是，由于CityExpert 中的字段太多，我不想选择所有字段。因此，我将查询更改如下：

"select " +
        "e.user.name, " +
        "e.user.email, " +
        "e.documents " +
"from " +
        "CityExpert e " +
"where " +
        "( (lower(e.user.name) like concat('%', lower(?1), '%') or e.user.name is null) or ?1 = '' ) " +
        "and " +
        "( (lower(e.user.phone) like concat('%', lower(?2), '%') or e.user.phone is null) or ?2 = '' ) "

但是，显然我们不能在这样的实体中选择一对多字段，因为我在前面的查询中得到了一个MySQLSyntaxErrorException（请参阅this question）。因此，我将查询更改为以下内容：

"select " +
        "e.user.name, " +
        "e.user.email, " +
        "d " +
"from " +
        "CityExpert e " +
        "left join " +
        "e.documents d" +
"where " +
        "( (lower(e.user.name) like concat('%', lower(?1), '%') or e.user.name is null) or ?1 = '' ) " +
        "and " +
        "( (lower(e.user.phone) like concat('%', lower(?2), '%') or e.user.phone is null) or ?2 = '' ) "

但是，这次结果变成了List<Object[]>，而不是List<CityExpert>。

我创建了以下 DTO：

public class CityExpertDTO {

    private String name;
    private String email;
    private List<CityExpertDocument> documents;

}

但是，我不知道应该如何将 Hibernate 返回的结果映射到List<CityExpertDTO>。我的意思是，我可以手动执行此操作，但肯定有 Hibernate 提供的自动化解决方案。

我正在使用 Spring Data JPA 并使用 HQL，如下所示：

public interface CityExpertRepository extends JpaRepository<CityExpert, Long> {

    @Query(
            "select " +
                    "e " +
            "from " +
                    "CityExpert e " +
            "where " +
                    "( (lower(e.user.name) like concat('%', lower(?1), '%') or e.user.name is null) or ?1 = '' ) " +
                    "and " +
                    "( (lower(e.user.phone) like concat('%', lower(?2), '%') or e.user.phone is null) or ?2 = '' ) "
    )
    Set<CityExpert> findUsingNameAndPhoneNumber(String name,
                                                String phoneNumber);

}

如何将结果映射到CityExpertDTO？

Answer 1

表关系

假设我们有以下post 和post_comment 表，它们通过post_comment 表中的post_id 外键列形成一对多关系。

SQL 投影

考虑到我们的用例只需要从post 表中获取id 和title 列，以及从post_comment 表中获取id 和review 列，我们可以使用以下 JPQL 查询来获取所需的投影：

select p.id as p_id, 
       p.title as p_title,
       pc.id as pc_id, 
       pc.review as pc_review
from PostComment pc
join pc.post p
order by pc.id

在运行上面的投影查询时，我们得到以下结果：

| p.id | p.title                           | pc.id | pc.review                             |
|------|-----------------------------------|-------|---------------------------------------|
| 1    | High-Performance Java Persistence | 1     | Best book on JPA and Hibernate!       |
| 1    | High-Performance Java Persistence | 2     | A must-read for every Java developer! |
| 2    | Hypersistence Optimizer           | 3     | It's like pair programming with Vlad! |

DTO 投影

但是，我们不想使用基于表格的 ResultSet 或默认的 List<Object[]>JPA 或 Hibernate 查询投影。我们希望将上述查询结果集转换为PostDTO 对象中的List，每个这样的对象都有一个comments 集合，其中包含所有关联的PostCommentDTO 对象：

我们可以使用 Hibernate ResultTransformer，如下例所示：

List<PostDTO> postDTOs = entityManager.createQuery("""
    select p.id as p_id, 
           p.title as p_title,
           pc.id as pc_id, 
           pc.review as pc_review
    from PostComment pc
    join pc.post p
    order by pc.id
    """)
.unwrap(org.hibernate.query.Query.class)
.setResultTransformer(new PostDTOResultTransformer())
.getResultList();

assertEquals(2, postDTOs.size());
assertEquals(2, postDTOs.get(0).getComments().size());
assertEquals(1, postDTOs.get(1).getComments().size());

PostDTOResultTransformer 将定义 Object[] 投影和包含 PostCommentDTO 子 DTO 对象的 PostDTO 对象之间的映射：

public class PostDTOResultTransformer 
        implements ResultTransformer {

    private Map<Long, PostDTO> postDTOMap = new LinkedHashMap<>();

    @Override
    public Object transformTuple(
            Object[] tuple, 
            String[] aliases) {
            
        Map<String, Integer> aliasToIndexMap = aliasToIndexMap(aliases);
        
        Long postId = longValue(tuple[aliasToIndexMap.get(PostDTO.ID_ALIAS)]);

        PostDTO postDTO = postDTOMap.computeIfAbsent(
            postId, 
            id -> new PostDTO(tuple, aliasToIndexMap)
        );
        
        postDTO.getComments().add(
            new PostCommentDTO(tuple, aliasToIndexMap)
        );

        return postDTO;
    }

    @Override
    public List transformList(List collection) {
        return new ArrayList<>(postDTOMap.values());
    }
}

aliasToIndexMap 只是一个小实用程序，它允许我们构建一个 Map 结构，该结构将列别名和列值所在的索引关联到 Object[] tuple 数组中：

public  Map<String, Integer> aliasToIndexMap(
        String[] aliases) {
    
    Map<String, Integer> aliasToIndexMap = new LinkedHashMap<>();
    
    for (int i = 0; i < aliases.length; i++) {
        aliasToIndexMap.put(aliases[i], i);
    }
    
    return aliasToIndexMap;
}

postDTOMap 是我们将存储所有PostDTO 实体的地方，这些实体最终将由查询执行返回。我们使用postDTOMap 的原因是父行在每个子记录的 SQL 查询结果集中都有重复。

computeIfAbsent 方法允许我们创建一个 PostDTO 对象，前提是 postDTOMap 中已经没有现有的 PostDTO 引用。

PostDTO 类有一个构造函数，可以使用专用列别名设置id 和title 属性：

public class PostDTO {

    public static final String ID_ALIAS = "p_id";
    
    public static final String TITLE_ALIAS = "p_title";

    private Long id;

    private String title;

    private List<PostCommentDTO> comments = new ArrayList<>();

    public PostDTO(
            Object[] tuples, 
            Map<String, Integer> aliasToIndexMap) {
            
        this.id = longValue(tuples[aliasToIndexMap.get(ID_ALIAS)]);
        this.title = stringValue(tuples[aliasToIndexMap.get(TITLE_ALIAS)]);
    }

    //Getters and setters omitted for brevity
}

PostCommentDTO 以类似的方式构建：

public class PostCommentDTO {

    public static final String ID_ALIAS = "pc_id";
    
    public static final String REVIEW_ALIAS = "pc_review";

    private Long id;

    private String review;

    public PostCommentDTO(
            Object[] tuples, 
            Map<String, Integer> aliasToIndexMap) {
        this.id = longValue(tuples[aliasToIndexMap.get(ID_ALIAS)]);
        this.review = stringValue(tuples[aliasToIndexMap.get(REVIEW_ALIAS)]);
    }

    //Getters and setters omitted for brevity
}

就是这样！

使用PostDTOResultTransformer，可以将 SQL 结果集转换为分层 DTO 投影，使用起来非常方便，尤其是在需要编组为 JSON 响应时：

postDTOs = {ArrayList}, size = 2
  0 = {PostDTO} 
    id = 1L
    title = "High-Performance Java Persistence"
    comments = {ArrayList}, size = 2
      0 = {PostCommentDTO} 
        id = 1L
        review = "Best book on JPA and Hibernate!"
      1 = {PostCommentDTO} 
        id = 2L
        review = "A must read for every Java developer!"
  1 = {PostDTO} 
    id = 2L
    title = "Hypersistence Optimizer"
    comments = {ArrayList}, size = 1
      0 = {PostCommentDTO} 
       id = 3L
       review = "It's like pair programming with Vlad!"