How to create a RestAPIs with JPA relational entities

Question

I have a problem. I don't know how to create an API when I have other entities. I work with Postman and when I do a request to see all projects from the database I want to receive the entities also.

For example this is my entity:

@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;

This is Gate entity

@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;

Threshold entity

@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;

Controller

@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);
    }

    @PostMapping
 //   @Consumes(MediaType.APPLICATION_JSON_VALUE)
    public Project saveProject(@RequestBody Project newProj) {
        return projectService.saveProject(newProj);
    }
}

When I do a Get request in Postman, I receive this output for example:

 {
        "projectId": 1,
        "projectName": "jenkins",
        "dgnumber": 1
    },

I expect to receive information about gate and threshold also. I don't understand how to do this things more exact.

`@JsonBackReference` is not appropriate for ManyToMany relations. https://stackoverflow.com/q/41260579/2224047 — Nikolai Shevchenko, Sep 26 '18 at 16:41

score 1 · Answer 1 · answered Sep 26 '18 at 18:26

related entities are not loaded by default in JPA. you have to define fetch = FetchType.EAGER in @ManyToMany relationship

@ManyToMany(fetch = FetchType.EAGER)
@JoinTable(name = "project_gate_relation", joinColumns = @JoinColumn(name = "proj_id"), inverseJoinColumns = @JoinColumn(name = "gate_id"))
@JsonBackReference
private  List<Gate> gates;

@ManyToMany(fetch = FetchType.EAGER)
@JoinTable(name = "project_threshold_relation", joinColumns = @JoinColumn(name = "proj_id"), inverseJoinColumns = @JoinColumn(name = "thresholdgates_id"))
@JsonBackReference
private  List<Threshold> thresholds;

score 0 · Accepted Answer · answered Sep 26 '18 at 15:19

Data associated with a @ManyToMany is by default lazily loaded. You need to specify that you want in eagerly loaded (if you are using spring-data, entity graphs can be used for that).

Please see the link for details: Spring Data JPA And NamedEntityGraphs

How to create a RestAPIs with JPA relational entities

2 Answers2