Update a Record if exists, else insert a new record in Room

Question

I am new to Room, Rxjava and other android architecture components. I am trying to update/insert 2 records (If row already exists,update it. otherwise insert a new row.) I have tried to do it in following way. But, I didn't work.

GameActivity:

public class GameActivity extends AppCompatActivity {

...

    public void onGameWinnerChanged(Player winner) {


    mDisposable.add(gameViewModel.updateDb(winner)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
            .subscribe(() -> Log.e("Success!!!", "updated records")
                    , throwable -> {
                throwable.printStackTrace();
            }));        
      }
}

GameViewModel:

public class GameViewModel extends ViewModel {

    ...

    public Completable updateDb(Player winner) {

        return Completable.fromAction(() -> {
            updateWinner(winner);
            Player loser = game.player1 == winner ? game.player2 : game.player1;
            updateLoser(loser);
        });
    }

    private void updateLoser(Player loser) {

           User user = userDataSource.getSingleRecordFromName(loser.name);
        if (user != null) {
            user.loss++;
            userDataSource.updateRecord(user);
        } else {
            user = new User(loser.name, "", 0, 0, 1);
            userDataSource.insertOrUpdateUser(user);
        }
    }

    private void updateWinner(Player winner) {

      User user = userDataSource.getSingleRecordFromName(winner.name);
      if (user != null) {
            user.wins++;
            userDataSource.updateRecord(user);
      } else {
            user = new User(winner.name, "", 0, 1, 0);
            userDataSource.insertOrUpdateUser(user);
      }
   }
}

LocalUserDataSource

public class LocalUserDataSource implements UserDataSource {

    private DaoAccess daoAccess;

    public LocalUserDataSource(DaoAccess daoAccess){
        this.daoAccess=daoAccess;
    }

    @Override
    public Flowable<List<User>> getUsers() {
        return daoAccess.fetchAllData();
    }

    @Override
    public void insertOrUpdateUser(User user) {
        Log.e("local user ds","insertOrUpdateUser");
        daoAccess.insertOnlySingleRecord(user);
    }

    @Override
    public User getSingleRecordFromName(String strName) {
        return daoAccess.getSingleRecord(strName);
    }


    @Override
    public void updateRecord(User user) {
        daoAccess.updateRecord(user);
    }

    @Override
    public void deleteRecord(User user) {
        daoAccess.deleteRecord(user);
    }
}

DaoAccess

@Dao
public interface DaoAccess  {

    @Insert
    void insertOnlySingleRecord(User user);

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

    @Query("SELECT * FROM User WHERE name =:strName")
    User getSingleRecord(String strName);

    @Update
    void updateRecord(User user);

    @Delete
    void deleteRecord(User user);
}

The problem is, when I try to run,NullPointer exceptions occurs if I try to insert any user which is not in database in following line of GameViewModel class.

User user = userDataSource.getSingleRecordFromName(loser.name);

Can anyone tell me where I am wrong or what should I do?

Edit

What I want to do is first fetch a user if it is there in database, update increment or decrement the score (if winner then +1 the wins column if loser then +1 the loss column). In case the user does not exists in the table, then create a new entry.

Answer 1

your user Dao should be like below

@Dao
public interface DaoAccess  {

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insertOnlySingleRecord(User user);

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

    @Query("SELECT * FROM User WHERE lower(name) = lower(:strName) limit 1")
    User getSingleRecord(String strName);

    @Update
    void updateRecord(User user);

    @Delete
    void deleteRecord(User user);
}

your user Entity should be like

 @Entity(tableName = "user_table", indices = @Index(value = {"name"}, unique = true))
public class UserEntity {

    @PrimaryKey(autoGenerate = true)
    private int id;

    @ColumnInfo(name = "name")
    private String name;
 }