QSqlQuery::isValid returns false

[mismael85]

hello,
i am accessing a ms access database and i have the folowing problem.

if(query->isSelect())
QMessageBox::warning(this, "title", "select true"); //always returns true
query->last();
if(query->isValid())
QMessageBox::warning(this, "title", "true"); // always return false

when i execute last() function it does not return the last record and the query position is set to an invalid position.
why when i execute the last() function it does not return the required record?
and why it points to an invalid position?
thank you

[wysota]

What does QSqlQuery::isActive() return?

[mismael85]

What does QSqlQuery::isActive() return?

i tried it before and after the QSqlQuery::last()
and it always returns true

[wysota]

Can you provide the whole block of code used to operate on the query?

[mismael85]

that is the code i use

#include "shamelareader.h"
 
shamelaReader::shamelaReader(QWidget *parent)
	: QMainWindow(parent)
{
	mainText = new QTextEdit(this);
	setCentralWidget(mainText);
 
	db= QSqlDatabase::addDatabase("QODBC");
	db.setDatabaseName("DRIVER={Microsoft Access Driver (*.mdb)};FIL={MS Access};DBQ=c:\\book.mdb");
	db.open();
 
 
	createActions();
    createToolBar();
 
    query = new QSqlQuery();
    query->exec("SELECT name FROM employee");
	query->next();
	mainText->setText(query->value(0).toString());
 
}
void shamelaReader::createActions()
{
	nextAct = new QAction(tr("&Next"),this);
	connect(nextAct, SIGNAL(triggered()), this, SLOT(next()));
 
	prevAct = new QAction(tr("&previous"),this);
	connect(prevAct, SIGNAL(triggered()), this, SLOT(prev()));
 
	firstAct = new QAction(tr("first"), this);
	connect(firstAct, SIGNAL(triggered()), this, SLOT(first()));
 
	lastAct = new QAction(tr("last"), this);
	connect(lastAct, SIGNAL(triggered()), this, SLOT(last()));
 
 
}
 
void shamelaReader::createToolBar()
{
  toolBar = addToolBar(tr("Navigation"));
  toolBar->addAction(firstAct);
  toolBar->addAction(nextAct);
  toolBar->addAction(prevAct);
  toolBar->addAction(lastAct);
}
 
void shamelaReader::next()
{
 query->next();
 mainText->setText(query->value(0).toString());
}
 
void shamelaReader::prev()
{
	query->previous();
    mainText->setText(query->value(0).toString());
}
 
void shamelaReader::first()
{
    query->first();
    mainText->setText(query->value(0).toString());
}
 
void shamelaReader::last()
{
 
	query->last();
	if(query->isActive())
        QMessageBox::warning(this, "title", "Active true");
 
	if(query->isValid())
		QMessageBox::warning(this, "title", "true");
 
    mainText->setText(query->value(0).toString());
}

#include "shamelareader.h"

shamelaReader::shamelaReader(QWidget *parent)
	: QMainWindow(parent)
{
	mainText = new QTextEdit(this);
	setCentralWidget(mainText);
  
	db= QSqlDatabase::addDatabase("QODBC");
	db.setDatabaseName("DRIVER={Microsoft Access Driver (*.mdb)};FIL={MS Access};DBQ=c:\\book.mdb");
	db.open();
		
    
	createActions();
    createToolBar();

    query = new QSqlQuery();
    query->exec("SELECT name FROM employee");
	query->next();
	mainText->setText(query->value(0).toString());
	
}
void shamelaReader::createActions()
{
	nextAct = new QAction(tr("&Next"),this);
	connect(nextAct, SIGNAL(triggered()), this, SLOT(next()));

	prevAct = new QAction(tr("&previous"),this);
	connect(prevAct, SIGNAL(triggered()), this, SLOT(prev()));

	firstAct = new QAction(tr("first"), this);
	connect(firstAct, SIGNAL(triggered()), this, SLOT(first()));

	lastAct = new QAction(tr("last"), this);
	connect(lastAct, SIGNAL(triggered()), this, SLOT(last()));


}

void shamelaReader::createToolBar()
{
  toolBar = addToolBar(tr("Navigation"));
  toolBar->addAction(firstAct);
  toolBar->addAction(nextAct);
  toolBar->addAction(prevAct);
  toolBar->addAction(lastAct);
}

void shamelaReader::next()
{
 query->next();
 mainText->setText(query->value(0).toString());
}

void shamelaReader::prev()
{
	query->previous();
    mainText->setText(query->value(0).toString());
}

void shamelaReader::first()
{
    query->first();
    mainText->setText(query->value(0).toString());
}

void shamelaReader::last()
{
	
	query->last();
	if(query->isActive())
        QMessageBox::warning(this, "title", "Active true");

	if(query->isValid())
		QMessageBox::warning(this, "title", "true");

    mainText->setText(query->value(0).toString());
}

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[wysota]

What does query->last() return? Maybe the driver only supports sequential access? Have you verified that?

[mismael85]

query->last()
return true

Maybe the driver only supports sequential access? Have you verified that?

how can i know that the driver does not support sequential access

[wysota]

if last() returns true, then it means the query has been positioned correctly.

if(query->last(){
  QSqlRecord rec = query->record();
  qDebug("%d", rec.count());
}

if(query->last(){
  QSqlRecord rec = query->record();
  qDebug("%d", rec.count());
}

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What does this output? Remember to have console enabled for your project or else you won't see anything.

[mismael85]

excuse me but i dont know how to enable the console.
i am using visual studio 2005.
i hope, i am not wasting your time.
but i believe if i want to learn some thing i must start my project directly as sayed in Qt documentation .but on the road there are missed things.
thank you

[wysota]

If you're using a commercial licence then access your project options using the integration plugin and it should be there somewhere. If not, run your application from within VS debugger and it should display the message. As an alternative, prepare a small qmake project and include a CONFIG+=console line in it.

[mismael85]

i made as you said and i get the following output:
1
QSqlQuery::value: not positioned on a valid record

[wysota]

How many records does the table hold?

[mismael85]

i tried it with two databases
one of them contains three records
and the other one contains 90 records.

[wysota]

Could you prepare a minimal compilable example reproducing the problem?

[mismael85]

here, it is the source code .
http://www.filesend.net/download.php...df51c62f649b7c

please dont forget to put the sample database in the ( c:\ ) directory.
the database are contained in the Zip file and also the *.pro file.

[mismael85]

i tried the same code with Sqlite database and the program works fine.
but why does not it work with MS access?

[wysota]

As I (think I) already said, maybe Access only supports sequential access to data.

[mismael85]

do you mean that i can not deal with access databases with Qt?

[wysota]

No, I mean Access databases can't jump across the result set regardless if you use Qt or not. If you want to reach the last entry, you have to iterate each record one by one until you reach the end.

[mismael85]

but i have the program that deal with access database which is built using visual basic
and this program go through the database easy it make last , first, next and previous
and i want to rebuild this program using c++. it means that the database support non sequential access
as well as sequential.