Manual references support

问题

Consider the class:

class Person {
    String name;
    Person father;
    Person mother;
    List<Person> children;
}

Is there a way to indicate to jongo that father, mother and children should be manual references to other objects within the same collection instead of embedded objects?

Note: this is different from DBRefs.

回答1:

Not yet.

The easiest way is to issue a second query manually.

Nevertheless, you can propably create a Jackson de/serializer to fetch documents during unmarshalling process. Few months ago, we have created a spike for fetching DBRef during unmarhsalling, this code can help : Handle DBRef during (un)marshalling

Feel free to add a feature request

回答2:

I use a way to handle it. Not sure it's perfect but it works for myself.

Let's say you have these classes

public class MyObject {

    @Id
    private String id;
    private MyEmbeddedObject embeddedObject;

    public String getId() {
        return this.id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public MyEmbeddedObject getEmbeddedObject() {
        return this.embeddedObject;
    }

    public void setEmbeddedObject(MyEmbeddedObject embeddedObject) {
        this.embeddedObject = embeddedObject;
    }

}


public class MyEmbeddedObject {

    @Id
    private String id;

    public String getId() {
        return this.id;
    }

    public void setId(String id) {
        this.id = id;
    }

}

You want to store MyEmbeddedObject in collection "embed" and Object in collection "object" with a manual reference to "embed" collection ids. For example :

object : { id:1, embeddedObjectId: 99 }
embed : { id : 99 }

You can implement a custom serializer and deserializer for the MyEmbeddedObject

public class MyEmbeddedObjectSerializer extends JsonSerializer<MyEmbeddedObject> {

    @Override
    public void serialize(MyEmbeddedObject value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
        jgen.writeText(value.getId());
    }
}

public class MyEmbeddedObjectDeserializer extends JsonDeserializer<MyEmbeddedObject> {

    @Override
    public MyEmbeddedObject deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
        MyEmbeddedObject obj = new MyEmbeddedObject();
        obj.setId(jsonParser.getString());
        return new MyEmbeddedObject();

    }
 }

If your register your serializer in Jongo, the problem is that it will be used for MyEmbeddedObject as a member of MyObject but also for MyEmbeddedObject when it is stored by itself in the "embed" collection.

To apply the custom serizalizers/deserializers only when MyEmbeddedObject is embed in MyOBject, I use Mixin annotations.

public interface MyObjectMixIn {

    @JsonSerialize(using=MyEmbeddedObjectSerializer .class)
    public MyEmbeddedObject getEmbeddedObject();

    @JsonDeserialize(using=MyEmbeddedObjectDeserializer.class)
    public void setEmbeddedObject(MyEmbeddedObject embeddedObject);

}

Then define a module

public class MyModule extends SimpleModule {
    @Override
    public void setupModule(SetupContext context) {
        context.setMixInAnnotations(MyObject.class, MyObjectMixIn.class);
    }
}

Finaly register the module in Jongo

Jongo jongo = new Jongo(db, JacksonMapper.Builder().registerModule(new MyModule ()).build());

Now to use it for storage in Mongo

1. Store the embeddedObject by itself in collection "embed" to get an id
2. Be sure the embeddedObject member of object contains the id
3. Store the object in collection "object". Only the id of the embedded object will be stored

And to read datastore

1. Read the object from collection "object". The embeddedobject will be initialize only with its ID
2. If you need it, read the embeddedObject in collection "embed".

EDIT : to avoid lazy loading and load the complete object at deserialization, HandlerInstantiator object from Jackson is useful to customize serializer and deserializer construction mechanism.

Hope that helps.

来源：https://stackoverflow.com/questions/14939433/manual-references-support