I'm now confused about how to do a CRUD in a Rest API with Spring.

Let me explain, I have two routes to POST and PUT an entity. I created two DTOs createPostRequest and updatePostRequest for this. Because when adding, the properties cannot be null, while when updating they can (nulled properties are ignored).

Problem 1:

On my frontend, the user is asked to choose a list of tags from the database (multi select html). This is why createPostRequest has a tags property typed TagDTO. But, how can I use modelMapper to map, for example, the createPostRequest to the Post entity making sure that the tags exist in the database?

if for example a user try to insert a tag that does not exist, I was thinking of doing something like this:

postEntity.setTags(tagService.findAllByIds(postEntity.getTagsId()));

This makes a lot of repetition in the code, because between create and update method of my entity in service, there is a lot of identical code.

Problem 2:

Based on my problem 1, how can I easily map my two DTOs to the same entity without repeating the code 2x?

Code example - PostService (see comment)

This is an example for the update, but there will be almost identical code for the create, so how do I proceed?

@Transactional
public Post update(Integer postId, UpdatePostRequest request) {
    return Optional.ofNullable(this.getById(postId)).map(post -> {
      // here how to map non-null properties of my request 
            // into my post taking in consideration my comment above?   

      postDAO.save(post);

      return post;
    }).orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND));
}

================================

UPDATE:

As requested, found the code bellow.

The controller:

@RestController
@RequestMapping("/v1/posts")
public class PostController {
    RequestMapping(method = RequestMethod.POST, consumes = "application/json", produces = "application/json; charset=UTF-8")
    public ResponseEntity<Object> update(@Valid @RequestBody CreatePostRequest createPostRequest) {
        Post post = postService.create(createPostRequest);
        return new ApiResponseHandler(new PostDTO(post), HttpStatus.OK).response();
    }

    RequestMapping(value = "/{postId}", method = RequestMethod.PUT, consumes = "application/json", produces = "application/json; charset=UTF-8")
    public ResponseEntity<Object> update(@Valid @RequestBody UpdatePostRequest updatePostRequest, @PathVariable Integer postId) {
        Post post = postService.update(postId, updatePostRequest);
        return new ApiResponseHandler(new PostDTO(post), HttpStatus.OK).response();
    }
}

CreatePostRequest :

@Data
public class CreatePostRequest {
    @NotNull
    @Size(min = 10, max = 30)
    private Sting title;

    @NotNull
    @Size(min = 50, max = 600)
    private String description

    @NotNull
    @ValidDateString
    private String expirationDate;

    @NotNull
    private List<TagDTO> tags;

    public List<Integer> getTagIds() {
        return this.getTags().stream().map(TagDTO::getId).collect(Collectors.toList());
    }
}

UpdatePostRequest :

@Data
public class UpdatePostRequest {
    @Size(min = 10, max = 30)
    private Sting title;

    @Size(min = 50, max = 600)
    private String description

    @ValidDateString
    private String expirationDate;

    private List<TagDTO> tags;

    public List<Integer> getTagIds() {
        return this.getTags().stream().map(TagDTO::getId).collect(Collectors.toList());
    }
}

The service :

@Service
@Transactional
public class PostService {
    @Transactional
    public Post create(CreatePostRequest request) {
        ModelMapper modelMapper = new ModelMapper();
        Post post = modelMapper.map(request, Post.class);

        // map will not work for tags : how to check that tags exists in database ?

        return postDAO.save(post);
    }

    @Transactional
    public Post update(Integer postId, UpdatePostRequest request) {
        return Optional.ofNullable(this.getById(postId)).map(post -> {
            ModelMapper modelMapper = new ModelMapper();
            modelMapper.getConfiguration().setSkipNullEnabled(true);
            modelMapper.map(request, post);
            
            // map will not work for tags : how to check that tags exists in database ?
            
            postDAO.save(post);

            return post;
        }).orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND));
    }
}