POST Data from two or more classes

[Eleni Pap]

I am trying to make a new call POST on my API on my Spring boot project.
It is very simple when you use only an entity class. But how can you combine, two or more classes?

For example you have a class user: student or teacher .

I would like to have a register for course, in which POST call: create a new course, I need to use
name, lastname, from user class
student_code, gr_year from studentDetails
and last
titleOfCourse , date etc.

If I have
Student.class
Course.class

I need DTO class in order to use on controller and service,
StudentDTO.class
Course.DTO.class

Service:
CourseService.class
StudentService.class

On my services

  public course DTOtoEntity(CourseDTO dto) {
        Course in = new Course();
        in.setId(dto.getId());
        in.setCourse(dto.getCourse());
} 

Now, I don't know what to do.

public CourseDTO saveCours(CourseDTO course)  {
        Course newCourse=new Course();
        newCourse=courseService.dtoToEntity(course);//
//
        CourseDTO newCourseDTO=new CourseDTO(newCourse);

        return  newCourseDTO;
    }

       CourseDTO newCourseDTO=new CourseDTO(newCourse);
I try  to make another
public void saveCours(CourseDTO course)  {
//with the same way with other classes but I believe that is not the best solution. and as I try it, I didn't run.


Is there any way ?

[Stephan van Hulst]

Your controller action doesn't make sense to me, neither in purpose nor implementation.

You say you want to register for a course. I would write an action such as this:

@RequestMapping(
  path     = "courses",
  consumes = MediaType.APPLICATION_JSON,
  produces = MediaType.APPLICATION_JSON)
public final class CourseController {

  private final CourseRepository courses;

  private final RegistrationRepository registrations;

  @Autowired
  public RegistrationController(CourseRepository courses, RegistrationRepository registrations) {
    if (courses == null)
      throw new IllegalArgumentException("The course repository may not be null.");

    if (registrations == null)
      throw new IllegalArgumentException("The registration repository may not be null.");

    this.courses       = courses;
    this.registrations = registrations;
  }

  @PostMapping
  @UserIsStudent
  public CourseRegistrationDTO registerAuthenticatedStudent(
    @RequestParam            String  courseId,
    @AuthenticationPrincipal Student student
  ) {
    var course = courses.findById(courseId)
      .orElseThrow(() -> new ResponseStatusException(NOT_FOUND, "The course doesn't exist."));

    var registration = registrations.registerStudentForCourse(student.id(), course.id())
      .orElseThrow(() -> new ResponseStatusException(CONFLICT, "The student was already registered for the course."));

    return new CourseRegistrationDTO(registration);
  }
}

[Tim Holloway]

I'm not sure that "DTO" is appropriate. A Data Transfer Object was an essential part of JEE back when EJBs were specialized class objects, but JPA turned them into POJOs, and at that point the need of a separate transfer object to ferry data in and out of the persistence layer became unnecessary since the JPA entity object could serve  as its own DTO.

It's very common when working with databases to need to work with graphs of data. By "graph" in mathematical/computer science terms we're talking about a network of interconnected objects, not pie and bar charts. A tree structure, for example is what's known as a directed graph and if it doesn't cross-reference itself, it's further qualified as a Directed Acyclic Graph.

Indeed, the schema of a database is just one big graph, so when we work with part of it what we are actually dealing with is a sub-graph. Regardless, if you have an invoice header, line items and maybe inventory references, and you need to work on them as a unit, that's a graph. Or, as I sometimes call it, a working set, which is to say the set of entity objects that I'm going to be working with - adding, deleting, and/or modifiying elements therein, including possibly adjusting their relationships.

When I design a complex app, I have 2 persistence layers. The bottom layer is the Data Access Object (DAO) layer and the classes in it generally perform CRUD and search functions on individual tables. Or sometimes a parent/child set of tables. The layer above that is the Database Logic or Service Layer and it deals with working sets - retrieving, storing, and validating collections of related objects as a unit. Within these 2 layers I am transactional and connected to the database. Above the Service layer is the Business Logic layer, topped by the UI layer. Before passing data to those layers I detach the objects in the working set from the database and end the transaction that was started when the Service Layer method was invoked.

This keeps everything neatly organized. I don't accidentally modify the database in the application layers, since at that point I'm detached. All database operations go through a service method before reaching a DAO, no mater how simple, because a little extra overhead is well paid-for by knowing absolutely that nobody's reaching around corners to do things.

So that is how I can - and frequently do - POST data from multiple database Entity classes.

[Eleni Pap]

Thank you for your extensive reference on graph entity, you mean something like

@NamedEntityGraph(
  name = "post-entity-graph",
  attributeNodes = {
    @NamedAttributeNode("subject"),
    @NamedAttributeNode("user"),
    @NamedAttributeNode("comments"),
  }
)
@Entity
public class Post {...}

I didn't know about it.

In my example , the project is quite simple, so I haven't problems with DTOs. As, Stephan has mentioned this  part of code:

[ var course = courses.findById(courseId)
      .orElseThrow(() -> new ResponseStatusException(NOT_FOUND, "The course doesn't exist."));

help me to think how to continue my project.  

When I POST a new course, I should add pathVariable course_id, in order to connect all entities I want.


But you give me a motive for learning.
Thank you !!