如何从数据库表中进行嵌套的 JSON 响应

Question

我有 2 张桌子。

DATA

+------+--------+---------------------+------------+
| ID   | Name   | Desc                | Type       |
+------+--------+---------------------+------------+
| 1001 | Motor  | Main motor 50 volt  | Mechanical |
| 1002 | Nut    | 25 mm dia           | Mechanical |
| 1003 | Bolt   | Hexa bolt           | Mechanical |
| 1004 | Engine | 750cc liquid cooled | Mechanical |
| 1005 | Oil    | 1 liter             | NA         |
| 1006 | Filter | Airfilter           | NA         |
| 1007 | AC     | 1000w               | Electrical |
+------+--------+---------------------+------------+

Relationship

+-----------+-----------+----------+
| Parent_id | Relation  | Child_id |
+-----------+-----------+----------+
| 1001      | Accessory | 1002     |
| 1001      | Accessory | 1003     |
| 1001      | Service   | 1005     |
| 1001      | Service   | 1006     |
| 1004      | Accessory | 1002     |
| 1004      | Accessory | 1003     |
| 1004      | Service   | 1005     |
+-----------+-----------+----------+

当用户搜索1001时，我想提供以下响应

{
    "id": "1001",
    "name": "Motor",
    "desc": "Main motor 50 volt",
    "Accessory": [{
            "id": "1002",
            "name": "Nut"
        },
        {
            "id": "1003",
            "name": "Bolt"
        }
    ],
    "Service": [{
            "id": "1005",
            "name": "Oil"
        },
        {
            "id": "1006",
            "name": "Filter"
        }
    ]
}

这里的Accessory 和Service 来自relationship 表列relation。我对弹簧靴一无所知。在谷歌的帮助下，我完成了以下代码。这给了我一个级别的 json。但我无法找到如何创建这个嵌套的 JSON。请提供一些建议。

存储库：

@Repository
public interface MyDataRepo extends JpaRepository<Items, String> {

    @Query(value="SELECT D.id,D.name,D.desc,R.relation,R.child_id as childid,DC.name as childname
                  FROM DATA D 
                  JOIN RELATIONSHIP R ON D.ID=R.PARENT_ID 
                  JOIN DATA DC ON DC.id=R.CHILD_ID
                  WHERE D.ID=?1",nativeQuery=true)
    List<Data> findAllCategory(String id);

    public static interface Data {
      String getid();
      String getname();
      String getdesc();
      String getrelation();
      String getchildid();
      String getchildname();
    }
}

服务：

public List<Data> getMyData(String id) {
    return repo.findAllCategory(id);
}

控制器：

@GetMapping("/getData/{id}")
public ResponseEntity<List<Data>> retrieveData(@PathVariable String id) {
    List<Data> stud = service.getMyData(id);
    return ResponseEntity.ok().body(stud);
}

Answer 1

由于Relation字段是动态的，我们无法使用POJO来生成相关对象，所以我选择了一个Map对象来生成最终结果。

@GetMapping("/getData/{id}")
public ResponseEntity<Map<String, Object>> retrieveData(@PathVariable("id") Long id) {
    List<MyDataRepo.Data> stud = myDataRepo.findAllCategory(id);

    Map<String, Object> parent = new HashMap<>();
    parent.put("id", stud.get(0).getid());
    parent.put("name", stud.get(0).getname());
    parent.put("desc", stud.get(0).getdesc());

    stud.forEach(d -> {
        String r = d.getrelation();

        Map<String, String> child = new HashMap<>();
        child.put("id", d.getchildid());
        child.put("name", d.getchildname());

        if (parent.containsKey(r)) {
            List<Map<String, String>> children = (List<Map<String, String>>) parent.get(r);
            children.add(child);
        } else {
            List<Map<String, String>> children = new ArrayList<>();
            children.add(child);
            parent.put(r, children);
        }
    });

    return ResponseEntity.ok().body(parent);
}

最后，我们得到了结果

{
  "id": "1001",
  "name": "Motor",
  "desc": "Main motor 50 volt",
  "Accessory": [
    {
      "id": "1002",
      "name": "Nut"
    },
    {
      "id": "1003",
      "name": "Bolt"
    }
  ],
  "Service": [
    {
      "id": "1005",
      "name": "Oil"
    },
    {
      "id": "1006",
      "name": "Filter"
    }
  ]
}

Answer 2

为了构建嵌套投影，您应该在实体之间建立关系，例如：

@Entity
@Table(name = "Data")
public class Data {

    @Id
    private long id;

    @Column(name = "name")
    private String name;

    @Column(name = "desc")
    private String desc;

    @OneToMany(mappedBy = "parentId", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    private Set<Relationship> relationship;

    ...getters/setters...
}

@Entity
@Table(name = "Relationship")
@IdClass(RelationshipId.class)
public class Relationship {

    @Id
    @Column(name = "parent_id")
    private long parentId;

    @Id
    @Column(name = "relation")
    private String name;

    @Id
    @Column(name = "child_id")
    private long childId;

    @OneToOne
    @JoinColumn(name = "child_id", unique = true, nullable = false, updatable = false, insertable = false)
    private Data child;

    ...getters/setters...
}

这是复合键的 id 类：

public class RelationshipId implements Serializable {

    private long parentId;

    private String name;

    private long childId;

    // default constructor

    public RelationshipId(long parentId, String name, long childId) {
        this.parentId = parentId;
        this.name = name;
        this.childId = childId;
    }

    // equals() and hashCode()
}

之前，我检查了两个选项：@IdClass 和@EmbeddedId。 我认为为您使用 @IdClass 注释的更合适的方法。 详细说明：https://www.baeldung.com/jpa-composite-primary-keys

以下预测：

public interface DataView {

    String getId();

    String getName();

    String getDesc();

    List<RelationshipView> getRelationship();

}

public interface RelationshipView {

    String getId();

    String getName();

    String getChildId();

    DataItemView getChild();

}

public interface DataItemView {

    String getId();

    String getName();

    String getDesc();

}

在 repo 中，您可以通过单个查询接收所有需要的数据，并将数据重组到适当的 DTO 视图中：

public interface DataRepo extends JpaRepository<Data, String> {

    @Query(value = "SELECT d from Data d " +
            "left join fetch d.relationship as r " +
            "left join fetch r.child as c " +
            "where d.id=:id")
    DataView getDataViewById(long id);

}

@Service
public class DataService {

    @Autowired
    private DataRepo dataRepo;

    public DataDto getMyData(long id) {
        DataView data = dataRepo.getDataViewById(id);
        DataDto dataDto = new DataDto();
        dataDto.setId(data.getId());
        dataDto.setName(data.getName());
        dataDto.setDesc(data.getDesc());

        for (RelationshipView rs : data.getRelationship()) {
            DataItemView dataItemView = rs.getChild();
            switch (rs.getName()) {
                case "Accessory":
                    dataDto.getAccessory().add(new DataItemDto(dataItemView.getId(), dataItemView.getName()));
                    break;
                case "Service":
                    dataDto.getService().add(new DataItemDto(dataItemView.getId(), dataItemView.getName()));
                    break;
            }

        }

        return dataDto;
    }

}

DTO 的类：

public class DataDto {
    String id;
    String name;
    String desc;
    List<DataItemDto> accessory = new LinkedList<>();
    List<DataItemDto> service = new LinkedList<>();
    ...getters/setters...
}

public class DataItemDto {
    String id;
    String name;

    public DataItemDto(String id, String name) {
        this.id = id;
        this.name = name;
    }
    ...getters/setters...
}