Deserializing from JSON using foreign key

Question

I have a many to one relationship: A *<-->1 B and I want to deserialize A from a JSON having B's primary key (B exists in db with that primary key):

{
    "b": 1
}

I have tried the following:

@Entity
@Table(name = "table_a")
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public class A implements Serializable {

    @JsonIgnore
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "b", unique = true, nullable = false)
    private B b;
}

and

@Entity
@Table(name = "table_b")
public class B implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;

    @OneToMany(mappedBy = "b")
    private List<A> a = new ArrayList<>();
}

but object A is created with b = null. How can I deserialize A with b property correctly instantiated from db?

Note: I am using Jackson version 2.6.1.

where do you want to get the b info from? look in db and create custom object or just create empty been with id field? — varren, Oct 09 '17 at 19:55

score 2 · Accepted Answer · answered Oct 10 '17 at 07:09

You have several options and here is similar question :

@JsonCreator factory in B class (More info)
Custom deserializer

Custom ObjectIdResolver for @JsonIdentityInfo like

private class MyObjectIdResolver implements ObjectIdResolver {
    private Map<ObjectIdGenerator.IdKey,Object> _items  = new HashMap<>();

    @Override
    public void bindItem(ObjectIdGenerator.IdKey id, Object pojo) {
        if (!_items.containsKey(id)) _items.put(id, pojo); 
    }

    @Override
    public Object resolveId(ObjectIdGenerator.IdKey id) {
        Object object = _items.get(id);
        return object == null ? getById(id) : object;
    }

    protected Object getById(ObjectIdGenerator.IdKey id){
        Object object = null;
        try {
            //can resolve object from db here
            //objectRepository.getById((Integer)idKey.key, idKey.scope)
            object = id.scope.getConstructor().newInstance();
            id.scope.getMethod("setId", int.class).invoke(object, id.key);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return object;
    }

    @Override
    public ObjectIdResolver newForDeserialization(Object context) {
        return new MyObjectIdResolver();
    }

    @Override
    public boolean canUseFor(ObjectIdResolver resolverType) {
        return resolverType.getClass() == getClass();
    }
}

And use it like this:

@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class,
        resolver = MyObjectIdResolver.class, 
        property = "id", scope = B.class)
public class B  {
   // ...
}

Here is your case related gist demo more broad github project with some serialization thoughts

Hi @varren, thanks for your very interesting answer. I have one question about it: how can I pass an object repository to the `ObjectIdResolver`? — y.luis.rojo, Oct 10 '17 at 11:34
There are lots of options to consider. 1) Inject it in ObjectIdResolver on ObjectMapper creation and not in annotation [like this](https://gist.github.com/ftagsold/38a92cba5467fe74ae6b) 2) use some static reference to your repository/service/some repository factory 3) probably just autowire it [like here](https://github.com/terrestris/shogun2/blob/master/src/shogun2-core/src/main/java/de/terrestris/shogun2/converter/PersistentObjectIdResolver.java#L35-L41) more info [here](https://github.com/terrestris/shogun2/search?utf8=%E2%9C%93&q=PersistentObjectIdResolver&type=) — varren, Oct 10 '17 at 14:30
@y.luis ppl say you can use `SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext(this);` similar to https://stackoverflow.com/a/21562939/1032167 I actually never had much lack with it. Can probably try something like https://gist.github.com/varren/c84da2f357a1d86b24bf137f3144cc3a#file-myobjectidresolverautowired-java — varren, Oct 10 '17 at 14:36

Deserializing from JSON using foreign key

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