QSqlQuery

[AlbertoN]

Hi all,

i can't understand where's my mistake.

#include <QApplication>
#include <QtSql>
#include <QtWidgets/QMessageBox>
 
bool m_hastransaction     = false;
bool m_startedtransaction = false;
QSqlDatabase db;
 
 
void connection()
{
    db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName(":memory:");
    m_startedtransaction = false;
    m_hastransaction     = false;
    if (!db.open()) {
        QMessageBox::critical(0, qApp->tr("Cannot open database"),
            qApp->tr("Unable to establish a database connection.\n"
                     "This example needs SQLite support. Please read "
                     "the Qt SQL driver documentation for information how "
                     "to build it.\n\n"
                     "Click Cancel to exit."), QMessageBox::Ok);
        return;
    }
    m_hastransaction = db.driver()->hasFeature(QSqlDriver::Transactions);
}
 
void startTransaction()
{
    if(m_hastransaction)
        m_startedtransaction =  db.transaction();
}
 
bool endTransaction(bool value)
{
    if(!m_hastransaction || !m_startedtransaction)
        return value;
    else{
        if(value)
        {
            db.commit();
        }else{
            db.rollback();
        }
        m_startedtransaction = false;
    }
    return value;
}
 
bool UnloggedQueryExec(QString query,bool setForward)
{//it doesn't work
    startTransaction();
    QSqlQuery q(query);//QSqlQuery q = QSqlQuery(query);//it's the same
    q.setForwardOnly(setForward);
    bool ret = q.exec();
    if(!ret){
        qDebug() << "*** UnloggedQueryExec: query failed: " << query << q.lastError().text();
    }else{
        qDebug() << "*** UnloggedQueryExec: query ok";	
	}
    return endTransaction(ret);
}
 
bool UnloggedQueryExec1(QString query,bool setForward)
{//it works
    startTransaction();
    QSqlQuery q;
    q.setForwardOnly(setForward);
    bool ret = q.exec(query);
    if(!ret){
        qDebug() << "*** UnloggedQueryExec1: query failed: " << query << q.lastError().text();
    }else{
        qDebug() << "*** UnloggedQueryExec1: query ok";	
	}
    return endTransaction(ret);
}
 
int main(int argc, char *argv[])
{
    QApplication a(argc,argv);
 
    connection();
    UnloggedQueryExec("CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))",true);
    UnloggedQueryExec1("CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))",true);
 
    return a.exec();
}

#include <QApplication>
#include <QtSql>
#include <QtWidgets/QMessageBox>

bool m_hastransaction     = false;
bool m_startedtransaction = false;
QSqlDatabase db;


void connection()
{
    db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName(":memory:");
    m_startedtransaction = false;
    m_hastransaction     = false;
    if (!db.open()) {
        QMessageBox::critical(0, qApp->tr("Cannot open database"),
            qApp->tr("Unable to establish a database connection.\n"
                     "This example needs SQLite support. Please read "
                     "the Qt SQL driver documentation for information how "
                     "to build it.\n\n"
                     "Click Cancel to exit."), QMessageBox::Ok);
        return;
    }
    m_hastransaction = db.driver()->hasFeature(QSqlDriver::Transactions);
}

void startTransaction()
{
    if(m_hastransaction)
        m_startedtransaction =  db.transaction();
}

bool endTransaction(bool value)
{
    if(!m_hastransaction || !m_startedtransaction)
        return value;
    else{
        if(value)
        {
            db.commit();
        }else{
            db.rollback();
        }
        m_startedtransaction = false;
    }
    return value;
}

bool UnloggedQueryExec(QString query,bool setForward)
{//it doesn't work
    startTransaction();
    QSqlQuery q(query);//QSqlQuery q = QSqlQuery(query);//it's the same
    q.setForwardOnly(setForward);
    bool ret = q.exec();
    if(!ret){
        qDebug() << "*** UnloggedQueryExec: query failed: " << query << q.lastError().text();
    }else{
        qDebug() << "*** UnloggedQueryExec: query ok";	
	}
    return endTransaction(ret);
}

bool UnloggedQueryExec1(QString query,bool setForward)
{//it works
    startTransaction();
    QSqlQuery q;
    q.setForwardOnly(setForward);
    bool ret = q.exec(query);
    if(!ret){
        qDebug() << "*** UnloggedQueryExec1: query failed: " << query << q.lastError().text();
    }else{
        qDebug() << "*** UnloggedQueryExec1: query ok";	
	}
    return endTransaction(ret);
}

int main(int argc, char *argv[])
{
    QApplication a(argc,argv);
    
    connection();
    UnloggedQueryExec("CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))",true);
    UnloggedQueryExec1("CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))",true);
	
    return a.exec();
}

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Output:
*** UnloggedQueryExec: query failed:  "CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))" "table tipolog already exists Unable to fetch row"
*** UnloggedQueryExec1: query ok

Output:
*** UnloggedQueryExec: query failed:  "CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))" "table tipolog already exists Unable to fetch row"
*** UnloggedQueryExec1: query ok

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EDIT: ops i forget pro file, even if it should not matter:

QT         += core widgets sql
CONFIG     += console
TARGET      = test
TEMPLATE    = app
 
SOURCES    += main.cpp 
 
OBJECTS_DIR = build/o
MOC_DIR     = build/moc
UI_DIR      = build/ui
RCC_DIR     = build/rcc
DESTDIR     = bin

QT         += core widgets sql
CONFIG     += console
TARGET      = test
TEMPLATE    = app

SOURCES    += main.cpp 

OBJECTS_DIR = build/o
MOC_DIR     = build/moc
UI_DIR      = build/ui
RCC_DIR     = build/rcc
DESTDIR     = bin

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Someone could be so kind to help me?
Thanks in advance,
Alberto

[ChrisW67]

What is unclear about the error message your database engine is giving you?

Output:
*** UnloggedQueryExec: query failed:  "CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))" 
"table tipolog already exists Unable to fetch row"

Output:
*** UnloggedQueryExec: query failed:  "CREATE TABLE log(id INTEGER PRIMARY KEY,descr varchar(30))" 
"table tipolog already exists Unable to fetch row"

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BTW: it helps if the code you post and the errors you present match

[AlbertoN]

Thanks for your replay.

Yes, furthermore It's unclear to me why it works if I create the query in this way:

QSqlQuery q;
q.exec(query);

QSqlQuery q;
q.exec(query);

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while it doesn't work if I create it in these other way:

QSqlQuery q(query);
QSqlQuery q = QSqlQuery(query);

QSqlQuery q(query);
QSqlQuery q = QSqlQuery(query);

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This is not so clear even because there is an example in qt docs like mine(http://qt-project.org/doc/qt-5.0/qts...y.html#details):

QSqlQuery query("SELECT country FROM artist");
     while (query.next()) {
         QString country = query.value(0).toString();
         doSomething(country);
     }

QSqlQuery query("SELECT country FROM artist");
     while (query.next()) {
         QString country = query.value(0).toString();
         doSomething(country);
     }

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Though the error message of database engine is not so clear as I never created log table before therefore It shouldn't happen that kind of error message.

[ChrisW67]

As described in the docs, QSqlQuery::QSqlQuery() with a string argument will create the query and execute it. Calling exec() a second time might have unintended consequences, like trying to create the same table again.

[AlbertoN]

Ops i miss it: ", it will be executed".

Thanks and sorry for noob question