[FIXED] Serialize model without relation

Issue

I have two models:

@Entity
class ModelA {
  ...
  @OneToMany(mappedBy = "modelA")
  @JsonManagedReference
  private List<ModelB> modelBs = new ArrayList<ModelB>();
  ...
}

@Entity
class ModelB {
  ...
  @ManyToOne
  @JoinColumn(name = "modela_id")
  @JsonBackReference
  private ModelA modela;
  ...
}

And a REST/Resource controller for ModelA which has a list method:

@GetMapping("list")
public @ResponseBody ArrayList<ModelA> list() {
  ArrayList<ModelA> modelas = new ArrayList();

  for (ModelA moa : modelARepository.findAll()) {
    modelas.add(moa);
  }

  return modelas;
}

Now by default the response from that controller will contain a JSON array of ModelA objects but those will also contain ModelB. Is there a way to pick which relations you want serialized and when ?

Coming from Django/Laravel I would be solving this with dedicated serializes for instance I would have a ModelAWithRelatiionsSerializer where I would serializer ModelA + all of it's relations or ModelAWithModelBSerializer which would return ModelA serialized with one level of ModelB etc.

I know I can juse ignore annotation to not include it but that's really not the same and I saw nothing regarding serializes in Spring packages. What's the best or optimal way of doing this ?

Solution

For anyone else who is moving from Laravel(PHP), Django(Python), Rails(Ruby):

I created a package serializers in which I have a serializer class for each of my models.

public class UserSerializer {

  // Default user values
  public interface User {}

  // User values with users posts
  public interface UserWithPosts extends User {}  

}

Now in your model what you do is annotate fields to which serializers they belong to:

@Entity  
class User {

  @Id
  @GeneratedValue
  @JsonView(UserSerializer.User.class)
  private Long id;

  @JsonView(UserSerializer.User.class)
  private String username;

  @JsonView(UserSerializer.User.class)
  private String email;

  @OneToMany(mappedBy = "user")
  @JsonManagedReference  // This prevents infinite loop  
  @JsonView(UserSerializer.UserWithPosts.class)  
  private List<Post> posts;

  // .. getters and setters bellow
}

In your controller you than annotate your methods with @JsonView(..) as to which serializer you want to use.

@RestController  
@RequestMapping("users")  
public class UsersController {

  private UserRepository userRepository;

  public UserController(UserRepository userRepository) {
    this.userRepository = userRepository;
  }  

  /**
   * In list method we only want users default values to keep the payload smaller.
   */
  @GetMapping("list")
  @JsonView(UserSerializer.User.class)  
  public @ResponseBody ArrayList<User> list() {
    ArrayList<User> users = new ArrayList<>();

    for (User user : userRepository.findAll()) {
      users.add(user);
    }

    return users;
  }


  /**
   * In this method we are only fetching a single user so payload is smaller so we can add the users posts as well. You could also toggle this with a url filter/parameter.  
   */
  @GetMapping("show")
  @JsonView(UserSerializer.UserWithPosts.class)  
  public @ResponseBody ArrayList<User> show(@RequestParam(name = "id", required = true) Long id) {
    return userRepository.findOne(id):
  }

}  

Configuration! This was pretty easy to understand however when I first did it none of the examples/tutorials told/showed that you need to update the configuration as well. If you don't do it than you will have a problem with serializing your relations the serializers will still work however you would get something like this in your JSON response:

...
"posts": [
  {},
  {},
  {},..
]
...

So it would return a JSON object for each post user has but no values to fix this i created config package added a WebConfiguration class:

@Configuration
@EnableWebMvc
public class WebConfiguration extends WebMvcConfigurerAdapter {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        ObjectMapper mapper = Jackson2ObjectMapperBuilder.json().defaultViewInclusion(true).build();
        converters.add(new MappingJackson2HttpMessageConverter(mapper));
    }

}  

You can also do this in XML configuration. But I personally prefer using Java class based configuration. It would be great if spring documentation included better explanations for newcomers.

Answered By - Sterling Duchess
Answer Checked By - Senaida (JavaFixing Volunteer)