Room returns incorrect initialized object from generated query - mysql

Question

I have three tables, one containing Cards, one containing CardDecks and third one implementing a many-to-many relation between the former two and additionally containg a symbol for every relation entry.
My task is to get three columns from the card-table and the symbol from the relation-table and save it in a data Object specifically designed for handling those inputs, the codition being, that all entries match the given deckId. Or in (hopefully correct) sql-language:
#Query("SELECT R.symbol, C.title, C.type, C.source " +
"FROM card_table C JOIN cards_to_card_deck R ON C.id = R.card_id"+
"WHERE R.card_deck_id = :cardDeckId")
LiveData<List<CardWithSymbol>> getCardsWithSymbolInCardDeckById(long cardDeckId);
But the room implementation class generates:
#Override
public LiveData<List<CardWithSymbol>> getCardsWithSymbolInCardDeckById(long
cardDeckId) {
final String _sql = "SELECT R.symbol, C.title, C.typ, C.source FROM
cards_to_card_deck R INNER JOIN card_table C ON R.card_id = C.id WHERE
R.card_deck_id = ?";
final RoomSQLiteQuery _statement = RoomSQLiteQuery.acquire(_sql, 1);
int _argIndex = 1;
_statement.bindLong(_argIndex, cardDeckId);
return new ComputableLiveData<List<CardWithSymbol>>() {
private Observer _observer;
#Override
protected List<CardWithSymbol> compute() {
if (_observer == null) {
_observer = new Observer("cards_to_card_deck","card_table") {
#Override
public void onInvalidated(#NonNull Set<String> tables) {
invalidate();
}
};
__db.getInvalidationTracker().addWeakObserver(_observer);
}
final Cursor _cursor = __db.query(_statement);
try {
final int _cursorIndexOfSymbol = _cursor.getColumnIndexOrThrow("symbol");
final List<CardWithSymbol> _result = new ArrayList<CardWithSymbol>(_cursor.getCount());
while(_cursor.moveToNext()) {
final CardWithSymbol _item;
final int _tmpSymbol;
_tmpSymbol = _cursor.getInt(_cursorIndexOfSymbol);
_item = new CardWithSymbol(_tmpSymbol,null,null,null);
_result.add(_item);
}
return _result;
} finally {
_cursor.close();
}
}
#Override
protected void finalize() {
_statement.release();
}
}.getLiveData();
}
Where
_item = new CardWithSymbol(_tmpSymbol,null,null,null);
should return my fully initialized object.
The CardWithSymbol class is declared as follows:
public class CardWithSymbol {
public int symbol;
public String cardName;
public String cardType;
public String cardSource;
public CardWithSymbol(int symbol, String cardName, String cardType, String cardSource){
this.symbol = symbol;
this.cardName = cardName;
this.cardType = cardType;
this.cardSource = cardSource;
}
And the types of the columns returned by the query are:
int symbol, String title, String type, String source
I already went through some debugging and the rest of the application works just fine. I can even read the symbol from the objects return by the query, but as mentioned above for some reason room ignores the other three parameters and just defaults them to null in the query-implementation.

Answer 1

So after some trial and error and reading through the dao-documentation once again i found my error:
When creating a class for handling subsets of columns in room, it is important to tell room which variable coresponds to which columns via #ColumnInfo(name = "name of the column goes here")-annotation.
So changing my CardWithSymbol class as follows solved the issue for me:
import android.arch.persistence.room.ColumnInfo;
public class CardWithSymbol {
#ColumnInfo(name = "symbol")
public int symbol;
#ColumnInfo(name = "title")
public String cardName;
#ColumnInfo(name = "type")
public String cardType;
#ColumnInfo(name = "source")
public String cardSource;
public CardWithSymbol(int symbol, String cardName, String cardType, String cardSource){
this.symbol = symbol;
this.cardName = cardName;
this.cardType = cardType;
this.cardSource = cardSource;
}
}