Insert successful but no data found

Entity class:

@Entity
public class User {
 @PrimaryKey
 public int uid;

 @ColumnInfo(name = "first_name")
 public String firstName;

 @ColumnInfo(name = "last_name")
 public String lastName;

 public User(int uid, String firstName, String lastName){
     this.uid = uid;
     this.firstName = firstName;
     this.lastName = lastName;
 }
}

Dao Interface:

@Dao
public interface UserDao {

 @Query("SELECT * FROM User")
 Flowable<List<User>> getAlls();

 @Insert
 Completable addData(User modelClass);
}

Database:

@Database(entities = {User.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {

 private static AppDatabase instance;

 static AppDatabase getDatabase(Context context) {
     if (instance == null) {
        instance = Room.databaseBuilder(context, AppDatabase.class, "your_db_name").createFromFile(new File("mypath")).build();
     }
     return instance;
 }

 public abstract UserDao userDao();
}

ViewModel:

public class RoomWithRxJavaViewModel extends AndroidViewModel {

    private AppDatabase appDatabase;

    public RoomWithRxJavaViewModel(@NonNull Application application) {
        super(application);
        appDatabase = AppDatabase.getDatabase(application);
    }

    public Flowable<List<User>> getList() {
        return appDatabase.userDao().getAlls().subscribeOn(Schedulers.computation())
                .observeOn(AndroidSchedulers.mainThread());
    }

    public Completable addData(User modelClass) {

        return Completable.fromAction(() -> appDatabase.userDao().addData(modelClass))
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread());
    }

}

Activity where I print results:

RoomWithRxJavaViewModel viewModel = new RoomWithRxJavaViewModel(this.getApplication());

Disposable disposable = viewModel.addData(new User(2, "btt", "bbb"))
.subscribe(
 () -> { //onSucess
   Toast.makeText(this, "Completed!", Toast.LENGTH_SHORT).show();
   Disposable subscribe = viewModel.getList()
   .subscribe(modelClasses ->
       {
         Toast.makeText(this, "RoomWithRx: " + modelClasses.size(), Toast.LENGTH_LONG).show();
       }, e -> Toast.makeText(this, "Error: " + e.getMessage(), Toast.LENGTH_LONG).show());
}, //onError
   throwable -> {
     Toast.makeText(this, "Error!" + throwable.getMessage(), Toast.LENGTH_SHORT).show();
  }
);

Creating a default user the insert is successful because I enter the onSucess function. I want to check the size of the updated User table but no matter how often I insert the user, the size remains 0. Why?

Your DAO:

@Insert
Single<Long> insert(User user);

Change Completable to Single as following:

public Single<Long> addData2(User user) {
    return appDatabase.userDao().addData(user)
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread());
}

And your modelview should look like this:

Disposable subscribe1 = viewModel.addData2(new User(4, "some", "user"))
 .subscribe(
  id -> { //onSucess },
  throwable -> { //onerror }
);