When to use CursorJoiner / MatrixCursor / MergeCursor?

Question

I\'m exploring different ways to get data elegantly from two or more joined tables.

I believe MergeCursor, (Android Developer Guide) seems to imply that

Answer 1

With regard to MatrixCursor, here's an example use.

This returns a decrypted version of the data (in this case just one column, but in the full version a number of columns are encrypted).

public MatrixCursor decyrptedCard(long cardid) {
    EncryptDecrypt ed = new EncryptDecrypt(mContext,
            LoginActivity.getCurrentUserPassWord(),
            MainActivity.mCurrentUserid);
    String[] mcsrcolumns = {
            DBCardsTableConstants.CARDID.getDBColumnName(),
            DBCardsTableConstants.CARDNAMEONCARD.getDBColumnName()
    };
    MatrixCursor cnvcsr = new MatrixCursor(mcsrcolumns,0);
    String whereclause = DBCardsTableConstants.CARDID.getDBColumnName() +
            "=?";
    String[] whereargs = {Long.toString(cardid)};

    Cursor basecsr = db.query(DBCardsTableConstants.CARDS.getDBTableName(),
            null,
            whereclause,
            whereargs,
            null,null,null,null);
    if (!basecsr.moveToFirst()) {
        cnvcsr.addRow(new Object[]{0L,"NOTACARD"});
        return cnvcsr;
    }

    cnvcsr.addRow(new Object[]{
            basecsr.getLong(
                    basecsr.getColumnIndex(
                            DBCardsTableConstants.CARDID.getDBColumnName()
                    )),
            ed.decrypt(
                    basecsr.getString(
                            basecsr.getColumnIndex(
                                    DBCardsTableConstants.CARDNAMEONCARD.getDBColumnName()
                            )
                    )
            )
    });
    basecsr.close();
    return cnvcsr;
}

In short it's little different to using a normal cursor, except you define the columns when you create an instance. You can then add rows with the addRow method.