QSqlQuery.exec(query) returns wrong character for unicode chars

i have a database in MS SQL server and my problem is unicode chars! "query.exec(str)" in my "getRecordByCondition" method returns "?" char for nvarchar fields! but it wroks correctly in "loadToGrid" method. here is my code:

void DataAccess::connect()
{
    db=QSqlDatabase::addDatabase("QODBC");
    db.setDatabaseName("TestDB");
}

void DataAccess::disConnect()
{
    QString connection;
    connection = db.connectionName();
    db.close();

}
QVariantList DataAccess::getRecordByCondition(QString tableName, QString condition)
{
    connect();
    QVariantList recordData;
    QString str= " select * from " + tableName + " where " + condition ;
    QSqlRecord record;
    QSqlQuery query(db);
    if(db.open())
    {

        // can run
        if(query.exec(str))
        {
            record=query.record();
            int cols=record.count();
            while (query.next())
            {
                for(int i=0;i<cols;i++)
                {
                    qDebug() << query.value(i); // ??????? ????
                    recordData << query.value(i);
                }
            }
            disConnect();
            //return 1;
        }
        else
        {
          qDebug() <<  "problem!" << db.lastError();
            disConnect();
            //return -1;
        }
    }
    else
    {

        qDebug() <<  "db can not open because:" << db.lastError().text();
        //return -2;
    }
   disConnect();
    return recordData;
}



void DataAccess::loadToGrid(QString tableName, QTableView &grid)
{
    connect();
    if(db.open())
    {
        //Load data
        QString qry="select * from " + tableName;
        QSqlRecord record;
        QSqlQuery query(db);
        if(query.exec(qry))
        {
             record=query.record();
             int cols=record.count();
             int rows=0;
             //make model
             QStandardItemModel *myModel=new QStandardItemModel(0,cols,grid.parent());
             //make headers
             for(int i=0;i<cols;i++)
             {
                 myModel->setHorizontalHeaderItem(i,new QStandardItem(record.fieldName(i)));
             }
             while (query.next()) {

                for(int i=0;i<cols;i++)
                {
                    myModel->setItem(rows,i,new QStandardItem(query.value(i).toString()));
                }
                rows++;
             }
             grid.setModel(myModel);

        }
    }
   disConnect();
}

i solved this, i only changed my connect method

void DataAccess::connect()
{

    if(!db.open())
    {
        db=QSqlDatabase::addDatabase("QODBC");
        db.setDatabaseName("TCOM");
    }
}