open existing sqlite3 database under QT - linux

Question

I am very new to QT and SQLite DBMS. I am trying to open an existing database created using "sqlite3" command-line program under ubuntu Linux. The same database I am trying to access under QT using the following code :
void MainWindow::func()
{
QSqlQuery query;
accounts_db = new QSqlDatabase();
*accounts_db = QSqlDatabase::addDatabase("QSQLITE");
perror("? ");
accounts_db->setDatabaseName("/home/user/xyz.db");
QSqlError *a = new QSqlError();
*a = accounts_db->lastError();
perror(a->text().toLatin1());
if (!accounts_db->open()) {
perror("database open error :");
}
if ( !accounts_db->isOpen() ) {
perror("database is not open");
}
query.exec("select accno,branchcode,fname,lname,curbalance,accdate from accounts");
while(query.next()) {
QString str = query.value(0).toString();
std::cerr << qPrintable(str) << std::endl;
}
end:
;
}
This fails with the following errors...
No such file or directory
: Invalid argument
QSqlQuery::exec: database not open
Notice that I get "No such file or directory" after adddatabase(), have no clue whatsoever which file is it talking about. Also notice that isOpen() and open() are returning "true" (???). The "database not open" error is from db.exec() call (...I suppose...).
In desperate need of guidance...

Answer 1

The constructor of QSqlQuery with no parameters uses the default database for your application. Maybe it is not set yet. Use the constructor specifying the database the query is required to use:
QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE", "connection_name");
// Open db...
QSqlQuery query(db);
if (!query.exec(...)) {
// ...
}
// ...
Pay attention to how you close the connection afterwards.
EDIT: This is a test I just wrote and is working on my system. You might want to try.
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QVariant>
int main(int argc, char *argv[])
{
// Create database.
QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE", "Connection");
db.setDatabaseName("/tmp/test.db");
if (!db.open()) {
qDebug("Error occurred opening the database.");
qDebug("%s.", qPrintable(db.lastError().text()));
return -1;
}
// Insert table.
QSqlQuery query(db);
query.prepare("CREATE TABLE IF NOT EXISTS test (id INTEGER PRIMARY KEY, text TEXT)");
if (!query.exec()) {
qDebug("Error occurred creating table.");
qDebug("%s.", qPrintable(db.lastError().text()));
return -1;
}
// Insert row.
query.prepare("INSERT INTO test VALUES (null, ?)");
query.addBindValue("Some text");
if (!query.exec()) {
qDebug("Error occurred inserting.");
qDebug("%s.", qPrintable(db.lastError().text()));
return -1;
}
// Query.
query.prepare("SELECT * FROM test");
if (!query.exec()) {
qDebug("Error occurred querying.");
qDebug("%s.", qPrintable(db.lastError().text()));
return -1;
}
while (query.next()) {
qDebug("id = %d, text = %s.", query.value(0).toInt(),
qPrintable(query.value(1).toString()));
}
return 0;
}

Answer 2

This is mostly guessing, since your code is wrong on quite a few things, including the error reporting.
The most likely problem is that your file path is simply not right, or the user you're running your application with does not have the appropriate permissions on the file and/or directory. (Note: files and directory are case sensitive in Linux.)
perror should only be used after calling a system function that actually failed and that sets errno when it does. Qt doesn't do that.
Please try running this, and update your question if you still cannot resolve your issue:
void MainWindow::func()
{
// Note: no pointer!
QSqlDatabase accounts_db = QSqlDatabase::addDatabase("QSQLITE");
accounts_db.setDatabaseName("/home/user/xyz.db");
if (!accounts_db.open())
{
qDebug() << "Could not open database file:";
qDebug() << accounts_db.lastError();
return;
}
// Note: don't construct queries before you have a database!
QSqlQuery query;
if (!query.exec("select accno,branchcode,fname,lname,curbalance,accdate from accounts"))
{
qDebug() << "Query failed:";
qDebug() << query.lastError();
return;
}
while(query.next()) {
QString str = query.value(0).toString();
std::cerr << qPrintable(str) << std::endl;
}
}
(I haven't even tried to compile this, so YMMV.)
Have a look at the SQL examples also, and look at how they handle all this there.

Answer 3

Alright, I created a brand new database file using sqlite3 command, now the same program in my question is working !!!.
I had also tried this before, but that time it did not work (...I hate it when that happens). I guess the previous file may be corrupted. But the previous database file could be accessed from cmdline sqlite3 program, so I was assuming that file was ok,...but apparently not.
Anyway, thanks a lot guys for giving me time, and very sorry if I wasted it :( .
I am marking this as an answer just for the sake of clarity that this question is answered. But its not exactly an answer (...because I still don't understand what was happening !? )
Thanks again...
EDIT :
Here is the code
QSqlError *a = new QSqlError();
accounts_db = new QSqlDatabase();
*accounts_db = QSqlDatabase::addDatabase("QSQLITE");
accounts_db->setDatabaseName("/home/user/test.db");
if ( !accounts_db->open() ) {
qDebug() << accounts_db->lastError();
qDebug() << "Could not open database file:";
}
QSqlQuery query;
if ( !(accounts_db->isOpen()) ) {
qDebug() << accounts_db->lastError();
qDebug() << ": Could not open database file:";
goto end; // quit if not successful
}
query.exec("select * from accounts");
while(query.next()) {
// loop for i columns
QString str = query.value(i).toString();
std::cerr << qPrintable(str) << std::endl ;
// loop
}
end:
;