[FIXED] JPA: Automatically create relations from JSON without having to parse manually

Issue

I'm developing a REST API with Spring Boot and I want to use my domain class as my request object (DTO). However, if I try to save an object with child entities, hibernate isn't able to create the correct relations.

Example

Assume the model of a Survey. Surveys contain Pages.

class Survey {
    @Id
    @GenericGenerator(name="uuid", strategy = "path.to.package.UUIDGenerator")
    @GeneratedValue(generator = "uuid")
    @JsonProperty(access = JsonProperty.Access.READ_ONLY)
    private UUID id;
    private String name;

    @OneToMany(cascade = {CascadeType.ALL}, mappedBy = "survey")
    private List<Page> pages = new ArrayList<>();
}

class Page {
    @Id
    @GenericGenerator(name="uuid", strategy = "path.to.package.UUIDGenerator")
    @GeneratedValue(generator = "uuid")
    private UUID id;
    private String label;

    @ManyToOne(cascade = {CascadeType.ALL})
    @JoinColumn(name = "survey_id")
    private Survey survey;
}

If I now want to create a new survey via the controller, I would do something like this

    @PostMapping
    public HttpEntity<SurveyDto> createSurvey(@RequestBody Survey request) {
        var s = surveyService.createSurvey(request);
        URI self = UriComponentsBuilder.fromPath(BASE_URL + "/{id}")
                .uriVariables(Map.of("id", s.getId()))
                .build().toUri();
        return ResponseEntity.created(self).body(s);

with a request that would look like this:

{
  "name": "survey1",
  "pages": [
    {
      "label": "First Page"
    },
    {
      "label": "Second Page"
    }
  ]
}

The JSON parser parses it correctly, however the references are not correct, as in: page.getSurvey() would be null. Therefore, hibernate can't set the relations (or at least that's what i think is happening)

My Question now is, if it is possible to achieve something, where I can just post a Survey as JSON and then just save that object, without having to manually map the JSON to a new Survey object where I manually set the pages array, since that would be a lot of mapping I'd have to do because of multiple child entities.

(First time posting a question, so if I missed any important information, feel free to request in the comments)

Solution

@M. Deinum had the right clue, by adding @JsonManagedReferenceto the list and @JsonBackReference to the survey field, Jackson knew how to correctly parse the object and therefore created the correct references.

Updated Example

class Survey {
    @Id
    @GenericGenerator(name="uuid", strategy = "path.to.package.UUIDGenerator")
    @GeneratedValue(generator = "uuid")
    private UUID id;
    private String name;

    @OneToMany(cascade = {CascadeType.ALL}, mappedBy = "survey")
    @JsonManagedReference
    private List<Page> pages = new ArrayList<>();
}

class Page {
    @Id
    @GenericGenerator(name="uuid", strategy = "path.to.package.UUIDGenerator")
    @GeneratedValue(generator = "uuid")
    private UUID id;
    private String label;

    @ManyToOne(cascade = {CascadeType.ALL})
    @JoinColumn(name = "survey_id")
    @JsonBackReference 
    private Survey survey;
}

Also, if you have multiple relations like these you have to give your references names with @JsonManagedReference(value = "survey-page") and @JsonBackReference(value = "survey-page)

Answered By - Christoph Kainz
Answer Checked By - Senaida (JavaFixing Volunteer)