Access parent from child in Realm for Android

Question

I am creating a chat application using Realm 1.2.0 for Android.

Here are two minimized models I created:

public class Room extends RealmObject {

    private String name;

    private RealmList<Message> messages;

}

public class Message extends RealmObject {

    @PrimaryKey
    private long id;

    private String text;

}

Using these models, I can get all messages in a specific room. The problem is, I also want to access the room from a message.

For example, this would be possible (accessing the childs from the parent):

room.getMessages()

But I also want to access the parent from the child, like this:

message.getRoom()

Is this possible in Realm without manually querying all the rooms again?

Answer 1

The concept is called backlink, and they're not in yet. In fact, they're in design stage or so, so it'll take a while before automatic inverse relationships are available.

So currently you'd need to maintain both sides of the relationship yourself.

public class Room extends RealmObject {

    private String name;

    private RealmList<Message> messages;

}

public class Message extends RealmObject {

    @PrimaryKey
    private long id;

    private String text;

    private Room room;
}

---

r.executeTransaction((realm) -> {
  Room room = realm.where(Room.class).equalTo("name", name).findFirst();
  Message message = new Message();
  message.setId(getNextId());
  message.setText(text);
  message.setRoom(room);
  message = realm.copyToRealmOrUpdate(message);
  room.getMessages().add(room);
}