Realm Java Sort By RealmList Link

If I have the following models:

class Conversation extends RealmObject {
    RealmList<Message> messages;
}

class Message extends RealmObject {
    long timestamp;
}

How can I say: give me all Conversations with the list of Message sorted, and the Conversations are sorted by the top item in messages?

I don't know, how achieve required result in your database scheme and without join operations (not supported yet¹).

But it can be achieved, if your database scheme will be migrated to this model:

class Conversation extends RealmObject {
    @PrimaryKey
    long id;

    public Long getId() {
        return id;
    }
}

class Message extends RealmObject {
    long timestamp;
    long conversationId;

    public long getConversationId() {
        return conversationId;
    }
}

Take note: using suggested models you need manually check for references consistency of "message -> conversation" relation.

Next, you can take required result by code below:

SortedSet<Message> sortedMessagesSet = new TreeSet<>(new Comparator<Message>() {
    @Override
    public int compare(Message message1, Message message2) {
        return (int) (message2.timestamp - message1.timestamp);
    }
});

List<Conversation> emptyConversations = new ArrayList<>();
// take "top" messages of each conversation
for (Conversation conversation : realm.where(Conversation.class).findAll()) {
    RealmResults<Message> sortedMessages = realm.where(Message.class).
                equalTo("conversationId", conversation.getId()).
                findAllSorted("timestamp", Sort.DESCENDING);
    // append latest (by time) message to the sorted messages set
    if (!sortedMessages.isEmpty()) sortedMessagesSet.add(sortedMessages.first());
    // remember conversations without messages
    else emptyConversations.add(conversation);
}

List<Conversation> sortedConversations = new ArrayList<>(sortedMessagesSet.size() + emptyConversations.size());
// take conversations sorted by messages
for (Message message : sortedMessagesSet)
        sortedConversations.add(realm.where(Conversation.class).
                equalTo("id", message.getConversationId()).findFirst());

// append empty conversations to the end
sortedConversations.addAll(emptyConversations);

1. Current version of realm is 2.3.0