”QSqlDatabasePrivate::removeDatabase: connection ‘test-connect‘ is still in use“数据库多次打开报错

数据库多次重复初始化会造成以下问题：

QSqlDatabasePrivate::removeDatabase: connection 'test-connect' is still in use, all queries will cease to work.
QSqlDatabasePrivate::addDatabase: duplicate connection name 'test-connect', old connection removed.

解决方法：

    if (!QSqlDatabase::contains("test-connect"))
        m_db = QSqlDatabase::addDatabase("QSQLITE","test-connect");  //第一次打开数据库时，连接数据库。
    else
        m_db = QSqlDatabase::database("test-connect");   //数据库已经打开后，再次连接时，获取句柄。