如何使用 JPA 关系实体创建 RestAPI

Question

我有个问题。当我有其他实体时，我不知道如何创建 API。我与 Postman 合作，当我请求查看数据库中的所有项目时，我也想接收实体。

例如，这是我的实体：

@Entity

@Table(name = "project")

public class Project {

    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    @Column(name = "proj_id")

    private int projectId;

    @Column(name = "project_name")

    private String projectName;

    @Column(name = "dg_number")

    private int dgNumber;

    @ManyToMany

    @JoinTable(name = "project_gate_relation", joinColumns = @JoinColumn(name = "proj_id"), inverseJoinColumns = @JoinColumn(name = "gate_id"))

    @JsonBackReference

    private  List<Gate> gates;

    @ManyToMany

    @JoinTable(name = "project_threshold_relation", joinColumns = @JoinColumn(name = "proj_id"), inverseJoinColumns = @JoinColumn(name = "thresholdgates_id"))

    @JsonBackReference

    private  List<Threshold> thresholds;

这是门实体

@Entity

@Table(name = "gate")

public class Gate {

    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    @Column(name = "gate_id")

    private int gateId;

    @Column(name = "gate_type")

    private String gateType;

    @Column(name = "gate_value")

    private float value;

阈值实体

@Entity

@Table(name = "threshold")

public class Threshold {

    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    @Column(name = "threshold_id")

    private int thresholdId;

    @Column(name = "threshold_value")

    private int thresholdValue;

    @Column(name = "threshold_type")

    private String thresholdType;

控制器

@RestController

@RequestMapping(ProjectController.PROJECT_URL)

public class ProjectController {

    public static final String PROJECT_URL = "/cidashboard/projects";

    @Autowired

    private final ProjectService projectService;

    public ProjectController(ProjectService projectService) {

        this.projectService = projectService;

    }

    @GetMapping

    public List<Project> getAllProjects(){

        return projectService.findAllProjects();

    }

    @GetMapping("/{id}")

    public Project getProjectById(@PathVariable int id) {

        return projectService.findProjectById(id);

    }

Answer 1

JPA 中默认不加载相关实体。你必须在@ManyToMany 关系中定义fetch = FetchType.EAGER@ManyToMany(fetch = FetchType.EAGER)@JoinTable(name = "project_gate_relation", joinColumns = @JoinColumn(name = "proj_id"), inverseJoinColumns = @JoinColumn(name = "gate_id"))@JsonBackReferenceprivate&nbsp; List<Gate> gates;@ManyToMany(fetch = FetchType.EAGER)@JoinTable(name = "project_threshold_relation", joinColumns = @JoinColumn(name = "proj_id"), inverseJoinColumns = @JoinColumn(name = "thresholdgates_id"))@JsonBackReferenceprivate&nbsp; List<Threshold> thresholds;

Answer 2

与 a 关联的数据@ManyToMany默认是延迟加载的。您需要在急切加载中指定您想要的（如果您使用的是 spring-data，则可以使用实体图）。