Something good to read about QSqlDatabase

[Morea]

Is there any good tutorial on QSqlDatabase?
I find it a little tricky to understand how to use it.

[spirit]

what are you confused?

[Morea]

Well it started with
db->isValid() // this works
db->isOpen() // this fails with segfault

so how do I check if I have a connection to the database?

Also, is it possible to create more persistant connections? I use PostgreSQL.

[spirit]

Well it started with
db->isValid() // this works
db->isOpen() // this fails with segfault

so how do I check if I have a connection to the database?

can you show compilable code with this seg-fault?



Also, is it possible to create more persistant connections? I use PostgreSQL.
yes, it is. you can create several database connection using,


QSqlDatabase QSqlDatabase::addDatabase ( const QString & type, const QString & connectionName = QLatin1String( defaultConnection ) )

but you must to set unique connectionName for needed connection

[Morea]

The code would be this (and the apply code is called when clicking on the button



#ifndef CONFIG_H
#define CONFIG_H

#include <QtSql>
#include <QtCore>
#include "ui_config.h"

class Config : public QDialog
{
Q_OBJECT
public:
Config(QWidget * parent = 0);
private:
Ui::Config ui;
QSqlDatabase* db;
QSettings* settings;
bool createDatabaseTable();
private slots:
void apply();
void noapply();
public slots:
};

#endif



#include "config.h"
#include <QtGui>
#include <QtCore>
#include "global.h"

Config::Config(QWidget *parent)
: QDialog(parent, 0)
{
setModal(true);
ui.setupUi(this);
connect(ui.apply,SIGNAL(clicked()),this,SLOT(apply ()));
connect(ui.noapply,SIGNAL(clicked()),this,SLOT(noa pply()));

settings = new QSettings(ORGANISATIONNAME,APPLICATIONNAME,this);
// Read configured values, if they exist
ui.hostname->setText(settings->value("hostname","").toString());
ui.port->setText(settings->value("port","").toString());
ui.database->setText(settings->value("database","").toString());
ui.username->setText(settings->value("username","").toString());
ui.password->setText(settings->value("password","").toString());

db->QSqlDatabase::addDatabase("QPSQL");
qDebug()<<db;
qDebug()<<db->isValid();
qDebug()<<"constructor done";
}

bool Config::createDatabaseTable()
{
bool ok=false;
if (! db->isOpen())
{
db->setHostName(settings->value("hostname","").toString());
QString tmp = settings->value("port").toString();
if (tmp != "");
db->setPort(tmp.toInt());
db->setDatabaseName(settings->value("database","").toString());
db->setUserName(settings->value("username","").toString());
db->setPassword(settings->value("password","").toString());
ok = db->open();
}
else
{
ok=true;
}
if (ok)
{
QSqlQuery result = db->exec("SELECT * FROM DomainTable");
if (result.isValid())
{
// table already exist.
return true;
}
else
{
// goahead and try to create the database table
return false;
}
}
else
{
QMessageBox::critical( this, "Failed to connect to database", "Edit connection settings again and apply");
return false;
}
}

void Config::noapply()
{
this->done(0);
}

void Config::apply()
{
settings->setValue("hostname",ui.hostname->text());
settings->setValue("database",ui.database->text());
settings->setValue("port",ui.port->text());
settings->setValue("username",ui.username->text());
settings->setValue("password",ui.password->text());
settings->sync();
qDebug()<<"Appply";
qDebug()<<"valid "<<db->isValid();
//qDebug()<<"open "<<db->isOpen();
qDebug()<<"continue";
bool ok=false;
if (! db->isOpen())
{
db->setHostName(settings->value("hostname","").toString());
QString tmp = settings->value("port").toString();
if (tmp != "");
db->setPort(tmp.toInt());
db->setDatabaseName(settings->value("database","").toString());
db->setUserName(settings->value("username","").toString());
db->setPassword(settings->value("password","").toString());
ok = db->open();
}
else
ok=true;
if (ok)
{
ok = createDatabaseTable();
if (ok)
this->done(1);
else
QMessageBox::critical( this, "Failed to create the database table", "Check database log");
}
else
{
QMessageBox::critical( this, "Failed to connect to database", "Edit connection settings");
}
// else continue to work...
}







<ui version="4.0" >
<class>Config</class>
<widget class="QDialog" name="Config" >
<property name="geometry" >
<rect>
<x>0</x>
<y>0</y>
<width>561</width>
<height>306</height>
</rect>
</property>
<property name="sizePolicy" >
<sizepolicy vsizetype="Fixed" hsizetype="Fixed" >
<horstretch>0</horstretch>
<verstretch>0</verstretch>
</sizepolicy>
</property>
<property name="windowTitle" >
<string>Database settings</string>
</property>
<widget class="QPushButton" name="apply" >
<property name="geometry" >
<rect>
<x>70</x>
<y>230</y>
<width>81</width>
<height>28</height>
</rect>
</property>
<property name="text" >
<string>Apply</string>
</property>
</widget>
<widget class="QPushButton" name="noapply" >
<property name="geometry" >
<rect>
<x>250</x>
<y>230</y>
<width>188</width>
<height>28</height>
</rect>
</property>
<property name="text" >
<string>Do NOT apply values above</string>
</property>
</widget>
<widget class="QLineEdit" name="username" >
<property name="geometry" >
<rect>
<x>100</x>
<y>130</y>
<width>451</width>
<height>24</height>
</rect>
</property>
</widget>
<widget class="QLabel" name="label" >
<property name="geometry" >
<rect>
<x>20</x>
<y>40</y>
<width>81</width>
<height>18</height>
</rect>
</property>
<property name="text" >
<string>Hostname</string>
</property>
</widget>
<widget class="QLabel" name="label_2" >
<property name="geometry" >
<rect>
<x>20</x>
<y>70</y>
<width>62</width>
<height>18</height>
</rect>
</property>
<property name="text" >
<string>Port</string>
</property>
</widget>
<widget class="QLabel" name="label_3" >
<property name="geometry" >
<rect>
<x>20</x>
<y>100</y>
<width>62</width>
<height>18</height>
</rect>
</property>
<property name="text" >
<string>Database</string>
</property>
</widget>
<widget class="QLabel" name="label_4" >
<property name="geometry" >
<rect>
<x>20</x>
<y>130</y>
<width>81</width>
<height>18</height>
</rect>
</property>
<property name="text" >
<string>Username</string>
</property>
</widget>
<widget class="QLineEdit" name="hostname" >
<property name="geometry" >
<rect>
<x>100</x>
<y>40</y>
<width>451</width>
<height>24</height>
</rect>
</property>
</widget>
<widget class="QLineEdit" name="port" >
<property name="geometry" >
<rect>
<x>100</x>
<y>70</y>
<width>113</width>
<height>24</height>
</rect>
</property>
</widget>
<widget class="QLineEdit" name="database" >
<property name="geometry" >
<rect>
<x>100</x>
<y>100</y>
<width>451</width>
<height>24</height>
</rect>
</property>
</widget>
<widget class="QLineEdit" name="password" >
<property name="geometry" >
<rect>
<x>100</x>
<y>160</y>
<width>451</width>
<height>24</height>
</rect>
</property>
</widget>
<widget class="QLabel" name="label_5" >
<property name="geometry" >
<rect>
<x>20</x>
<y>160</y>
<width>62</width>
<height>18</height>
</rect>
</property>
<property name="text" >
<string>Password</string>
</property>
</widget>
<widget class="QLabel" name="label_6" >
<property name="geometry" >
<rect>
<x>40</x>
<y>200</y>
<width>501</width>
<height>18</height>
</rect>
</property>
<property name="text" >
<string>Notice that hitting apply will access the database and create database tables!</string>
</property>
</widget>
</widget>
<tabstops>
<tabstop>hostname</tabstop>
<tabstop>port</tabstop>
<tabstop>database</tabstop>
<tabstop>username</tabstop>
<tabstop>password</tabstop>
<tabstop>apply</tabstop>
<tabstop>noapply</tabstop>
</tabstops>
<resources/>
<connections/>
</ui>

[spirit]

the first:
what is this?


...
db->QSqlDatabase::addDatabase("QPSQL");
...

I didn't see where you create object using operator new.

the second:
you shouldn't use pointer to database, because QSqlDatabase class is shared.
so, I suggest you to revise you code: refuse using pointer to database (but if you prefer
to use pointer then you must use new operator)

you can also not to keep variable for database like a class member, you can always get a needed database object using


QSqlDatabase QSqlDatabase::database ( const QString & connectionName = QLatin1String( defaultConnection ), bool open = true )


conclusion:
you can use the following code


void someObject::initDatabse
{
QSqlDatabese db = QSqlDatabse::addDatabase("QPSQL", "myConnection");
.....//do connection stuff
}

void someObject::execQuery(const QString &queryText)
{
QSqlDatabse db = QSqlDatabase::database("myConnection");
QSqlQuery query(db);
query.prepare(queryText);
....
}

[caduel]

in Qt4 QSqlDatabase is a value type: no need to handle with pointers like QSqlDatabase*
you did not initialize your db pointer (well, you should replace it by a QSqlDatabase anyway)


so replace QSqlDatabase *db; by QSqlDatabase db;
and db->QSqlDatabase::addDatabase("QPSQL"); by db = QSqlDatabase::addDatabase("QPSQL");

HTH

[Morea]

I'm currently editing

[spirit]

probaly you have set incorrect parameters for a database.


#include <QApplication>
#include <QtSql>

int main(int argc, char **argv)
{
QApplication app(argc, argv);

QSqlDatabase db2 = QSqlDatabase::addDatabase("QPSQL", "MyConnection");
db2.setHostName("localhost");
db2.setDatabaseName("curriculum");
db2.setUserName("postgres");
db2.setPassword("sysdba");

qDebug() << db2;//res == QSqlDatabase(driver=""QPSQL"", database=""curriculum"", host=""localhost"", port=-1, user=""postgres"", open=false)
qDebug() << QSqlDatabase::connectionNames();//res == MyConnection
QSqlDatabase db = QSqlDatabase::database("MyConnection");
qDebug() << db;//res == QSqlDatabase(driver=""QPSQL"", database=""curriculum"", host=""localhost"", port=-1, user=""postgres"", open=true)


return 0;
}

this code works fine.

[Morea]

with that code and setting port to -1 helped alot! :D