[Interest] how to properly close a SQLite database?

[Francisco Ares]

Thanks, Constantin.

Do you know if is it possible to have two QSqlDatabase objects (using
SQLite driver, of course), in the same program?  I think I will make some
tests with its "clone" function.

Francisco



2014-06-25 0:46 GMT-03:00 Constantin Makshin <cmakshin at gmail.com>:

> QSqlDatabase::close() closes the database backend connection but leaves
> QtSql one intact, i.e. even after close() your QSqlDatabase object
> occupies its slot in the [global] list of existing database objects. So
> in order to let QSqlDatabase::removeDatabase() do its job you have to
> destroy all QSqlDatabase objects referring to that connection.
>
> Here is a fragment from one of my applications:
>
> void Application::closeDatabase ()
> {
>     QString connectionName = m_db.connectionName();
>
>     m_db.close();
>     m_db = QSqlDatabase();
>     QSqlDatabase::removeDatabase(connectionName);
> }
>
> On 06/24/2014 09:25 PM, Francisco Ares wrote:
> > Hi,
> >
> > Before of anything else, thanks to anyone reading this message.  I'm
> > using Qt 4.8.5 and QtCreator.
> >
> > I use SQLite for data storage in an embedded system, and that is OK.
> > There is a class, named "CDataBase" that holds the "QSqlDatabase" object
> > and all of the "QSqlQuery" objects used on this program, and all access
> > to the database itself is made through member functions of this class,
> > providing a non-SQL interface for the rest of the program. The class has
> > an "init" function to receive the database file name:
> >
> > void CDataBase::init(QString fileName)
> > {
> >     mDB = QSqlDatabase::addDatabase("QSQLITE");
> >     mDB.setDatabaseName(fileName);
> >     mDB.open();
> >      ...
> > }
> >
> > Now I need to implement a backup for that data in a removable drive. For
> > that, I have created a second "CDataBase" object to handle the backup
> > data base file. The objects that use "CDataBase" have means implemented
> > to switch from a "CDataBase" object to another one (at least I believe
> > so, as I could not test it).
> >
> > When the program tries open the new object's database connection, the
> > program stops (in debug mode) on the first instruction of the "init"
> > function; hitting F10, a warning shows up on the program output pane:
> >
> > QSqlDatabasePrivate::removeDatabase: connection
> > 'qt_sql_default_connection' is still in use, all queries will cease to
> work.
> >
> > So, I've implemented a "finish()" member function for this class:
> >
> > void CDataBase::finish()
> > {
> >     if (mDB.isOpen())
> >     {
> >         mDB.commit();
> >         mDB.close();
> >         mDB.removeDatabase(mDB.connectionName());
> >     }
> > }
> >
> > So, when I try this:
> >
> >     mDataBaseWork->finish();
> >
> >
> > the same warning message shows up on trying to execute the
> > "removeDatabase" instruction.
> >
> > All the QSqlQueries are temporary, they are all dead at this point. I
> > even tried to do:
> >
> > {
> >     QSqlQuery query (mDB, query);
> >     ...
> >     ...
> >     query.finish();
> >     query.clear();
> > }
> >
> > And it didn't work as well.
> >
> > What am I doing wrong?
> >
> > Thanks again,
> > Francisco
>
>
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