[Interest] Close DB connection when not actively using?

Tue Aug 21 23:35:55 CEST 2018

Ok, so as a test, I threw together a basic project, consisting of just the default main window and boiler plate code created by Qt creator. I then modified main.cpp to be the following:

#include "mainwindow.h"
#include <QApplication>
#include <QSqlDatabase>

bool setupTestConnection(){
    QSqlDatabase db=QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("/tmp/testDb.db");
    bool result=db.open();
    db.close();
    return result;
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    bool canOpen=setupTestConnection();
    //TODO: if can't open, give appropriate error and terminate.

    qDebug("Call Result: %i, DB is STILL open: %i",canOpen,QSqlDatabase::database(QSqlDatabase::defaultConnection,false).isOpen());

    MainWindow w;
    w.show();

    return a.exec();
}

And as expected (since I called close()), the debug line did show it was closed. Then in the main window I added a QSqlTableModel member (which I called testTable), and put the following in the QMainWindow constructor to set up the table model:

testTable.setTable("test");
testTable.select();

So keeping it as basic as I can. Note that I never need to call QSqlDatabase::database(), so that particular link you sent doesn't help. When running this new application, lsof shows that the database is open at all times - so apparently the table model opened the connection and kept it open. So how is this one constructed wrong? I'm only opening the DB in main to make sure I can (which is kinda important), and I make sure to close it when I am done checking.

And BTW, I *have* read the manual. Repeatedly over the years. Obviously I have managed to miss the solutions you are seeing.
-----------------------------------------------
Israel Brewster
Systems Analyst II
5245 Airport Industrial Rd
Fairbanks, AK 99709
(907) 450-7293
-----------------------------------------------

[cid:6793ac7e-ede7-41ad-8860-67d4cac2b49a at flyravn.com]

[cid:bdd42050-cfb6-4772-9280-b5284ed00f17 at flyravn.com]

On Aug 21, 2018, at 1:08 PM, Roland Hughes <roland at logikalsolutions.com<mailto:roland at logikalsolutions.com>> wrote:

You opened the database in main. The function you called is so small it most likely was placed in-line by the compiler. This would be especially true if you did nothing with dbOpen.

You have no reason to open it there. Again, read the fine manual.

http://doc.qt.io/qt-5/qsqldatabase.html#database

Every time you call QSqlDatabase::database( "someConnectionName") it will open the database if it is not currently open. You should never open the database in main. Just set up all of the values for it.

On 08/21/2018 03:54 PM, Israel Brewster wrote:
Ok, maybe you can give me a pointer on what I'm doing wrong then. My code is structured like the following:

bool openDatabase(){
QSqlDatabase db=QSqlDatabase::addDatabase('QSQLITE');
db.setDatabaseName(<path_to_my_database_file>);
return db.open();
}

 int main(int argc, char *argv[])
{
QApplication a(argc, argv);
...
<various initialization code>
...
bool dbOpen=openDatabase();
...
<more initilization>
return a.exec();
}

And if I put a breakpoint at any point after the call to OpenDatabase, the database is open - it does not close, even though that QSqlDatabase object went out of scope. At no point in my application do I keep a copy of QSqlDatabase around in any of my classes - every call to QSqlDatabase is as a function local member, and as such goes out of scope (thereby theoretically calling the destructor) as soon as the function exits.

So what am I missing here?

-----------------------------------------------
Israel Brewster
Systems Analyst II
5245 Airport Industrial Rd
Fairbanks, AK 99709
(907) 450-7293
-----------------------------------------------

<image001.jpg>

<image002.jpg>

On Aug 21, 2018, at 12:31 PM, Roland Hughes <roland at logikalsolutions.com<mailto:roland at logikalsolutions.com>> wrote:

Why are you creating yet another class instead of properly structuring your application?

Take a gander at

http://www.logikalsolutions.com/wordpress/information-technology/qt-and-usb-pt-4/

for some ideas. You probably also want to find a copy of this book

http://www.theminimumyouneedtoknow.com/qt_book.html

Most importantly you need to read the fine manual.

http://doc.qt.io/qt-5/qsqldatabase.html#dtor.QSqlDatabase

QSqlDatabase::~QSqlDatabase()

Destroys the object and frees any allocated resources.

Note: When the last connection is destroyed, the destructor implicitly calls close<http://doc.qt.io/qt-5/qsqldatabase.html#close>() to release the database connection.

See also close<http://doc.qt.io/qt-5/qsqldatabase.html#close>().

If your database is open for the life of the application then the application has a failed architecture. Database connections aren't supposed to have actual life spans.

On 08/21/2018 03:08 PM, Israel Brewster wrote:

Ideally, what I'd have is a system where you set up the connection, and then it automatically opens/closes the connection when needed (i.e. when doing a SELECT, INSERT, UPDATE, etc). With my app, this would generally only be for a) periodic updates or b) in direct response to user input, so most of the time the connection could remain closed. To the best of my knowledge (correct me if I am wrong), this is not possible with the existing QSqlDatabase/ QSQLITE database driver classes.

--
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Logikal Solutions
(630) 205-1593

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Logikal Solutions
(630) 205-1593

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