Deserializing from JSON using foreign key

问题

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.

回答1:

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

来源：https://stackoverflow.com/questions/46650940/deserializing-from-json-using-foreign-key

标签

java

json

jpa

serialization

jackson