Question

I have a large object graph from which I would like to expose objects in a RESTful via a web-service. All objects feature an id-property, which serves as a primary key unique within the objects of that class. Further, all relationships in my object graph are bi-directional. When exposing an instance as a REST resource, I need all attributes of the instance to be written to JSON; however, I want the instances that are associated via relationships merely to be represented by their id, rather than having the entire object serialised to JSON.

Example: So, say a Person has a Dog and a Dog has a Person.

class Person {
  private Integer id;
  private String firstName;
  private String lastName;
  private Dog dog;

  // public getters/setters omitted
}

class Dog {
  private Integer id;
  private String name;
  private Person owner;

  // public getters/setters omitted
}

When serialising a given dog, I need the result to be:

{"id": 24, "name": "Fifo", "owner": 42}

And when serialising a given Person, I need the result to be:

{"id": 42, "firstName": "John", "lastName": "Blogs", "dog": 24}

rather than

{"id": 24, "name": "Fifo", "owner":
  {"id": 42, "firstName": "John", "lastName": "Blogs", "dog": 24}
}

and

{"id": 42, "firstName": "John", "lastName": "Blogs", "dog":
  {"id": 24, "name": "Fifo", "owner": 42}
}

respectively. I have played with the @JsonValue annotation in Jackson on the getId() method, but unfortunately, that applies always no matter whether the object it directly serialised or whether the object is reached via a relationship from another object.

I would have thought, that this use case is reasonably common. If this is not possible, what do other people do when exposing objects in an object graph via a RESTful API.

Thank you.

Answer 1

I can’t think of an obvious ways to do this with Jackson (there are many ways to override functionality, so there may be a way to do it with custom handlers, but that’s not simple), but it sounds like a good feature to bring on user/dev lists; others may well have implemented solutions, and it would be great to figure out a declarative solution here.

In fact I suspect that an annotation like @JsonKey could be added to handle simple cases where single property is used as the key. So a feature request might make sense.