RxJava and Sqlbrite - which operator to use

Question

I started using RxJava in combination with SqlBrite and I am having some problems with the zip operator.

Let's say I have 2 classes, User and Tweet .

 public class User { public long id; public List<Tweet> users; ... } public class Tweet { public long id; public User poster; ... } 

And their respective SQLite tables:

 user --------- id INTEGER PRIMARY KEY tweet --------- id INTEGER PRIMARY KEY poster_id INTEGER FOREIGN KEY(poster_id) REFERENCES user(id) 

and their corresponding SqlBrite DAO

UserDao:

 public class UserDao { private final BriteDatabase briteDb; private final TweetDAO tweetDao; ... public Observable<List<User>> getUsersWithTheirTweets() { Observable<User> usersObs = briteDb.createQuery("user", "SELECT * FROM user") .map(new Func1<SqlBrite.Query, List<User>>() { @Override public List<User> call(SqlBrite.Query query) { Cursor cursor = query.run(); List<User> result = new ArrayList<>(cursor.getCount()); while (cursor.moveToNext()) { User user = UserTable.parseCursor(cursor); result.add(user); } cursor.close(); return result; } }) // transform Observable<List<User>> into Observable<User> .flatMap(new Func1<List<User>, Observable<User>>() { @Override public Observable<User> call(List<User> users) { return Observable.from(users); } }); // combine each user with his tweets return Observable.zip(usersObs, usersObs.flatMap(new Func1<User, Observable<List<Tweet>>>() { @Override public Observable<List<Tweet>> call(User user) { return tweetDao.getTweetsByUser(user); } }), new Func2<User, List<Tweet>, User>() { @Override public User call(User user, List<Tweet> tweets) { user.tweets = tweets; return user; } }).toList(); } } 

TweetDAO:

 public class TweetDAO { private final BriteDatabase briteDb; ... public Observable<List<Tweet>> getTweetsForUser(final User user) { briteDb.createQuery("tweet", "SELECT * FROM tweet WHERE poster_id = ?", Long.toString(user.id)) .map(new Func1<SqlBrite.Query, List<User>>() { @Override public List<Tweet> call(SqlBrite.Query query) { Cursor cursor = query.run(); List<Tweet> result = new ArrayList<>(cursor.getCount()); while (cursor.moveToNext()) { Tweet tweet = TweetTable.parseCursor(cursor); tweet.user = user; result.add(tweet); } cursor.close(); return result; } }) } } 

As you can see in UserDao , I tried using a combination of zip and flatMap to populate the Tweet list for each User .

First question: is there a better way to do this?

Second question: the zip operator seems to hang forever and never ends ... I see that tweetDao.getTweetsByUser(user) is called, but Func zip never called ... Does anyone have an idea why?

Third question: is there a better way to convert an Observable<List<T>> to an Observable<T> other than using a combination of flatMap and from ?

Damn that RxJava is powerful, but the learning curve is pretty steep ...

Answer 1

Why not let the database handle the connection?

 SELECT * FROM user INNER JOIN tweet ON user._id = tweet.poster_id 

You may need to rename some columns if two tables have columns with the same name.

More about associations here