Properly close database handlers

[ruben.rodrigues]

Hi all!

I have a database application running in a remote server that therefore works as dataserver. The client send a request and the server replies to it after getting some info from the database. The code works but must be sharpen because every 20 connections( more or less) it fails to connect to the database. Is this message related to the problem? I get it every connection but it still works:

QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.
QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

how should I close the database properly?

[Lykurg]

Before you remove a connection make sure all QSqlQuerry/QSqlDatabase/QSql*** object's are destroyed of cleared. Then you can remove the database.

[ruben.rodrigues]

Before you remove a connection make sure all QSqlQuerry/QSqlDatabase/QSql*** object's are destroyed of cleared. Then you can remove the database.

Thanks for your answer!

I already tryed to clear close and delete all objects but the message keeps showing. I will post some of the code:

Header:

QSqlDatabase db;

QSqlDatabase db;

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Constructor:

db = QSqlDatabase::addDatabase("QMYSQL"); //select database type
db.setHostName("localhost"); //tell where the database is
db.setDatabaseName("qt");    //Database name
db.setUserName("user");       //username to connect to the database
db.setPassword("password");  //password from user

db = QSqlDatabase::addDatabase("QMYSQL"); //select database type
db.setHostName("localhost"); //tell where the database is
db.setDatabaseName("qt");    //Database name
db.setUserName("user");       //username to connect to the database
db.setPassword("password");  //password from user

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Function LoadProduct:

//check for errors on database connection
if (!db.open()) {
qDebug() << db.lastError();
         // return 4;
}
//set table to use
QString table = "mytable";
QSqlQuery query("USE " + table);
 
//Query to get table info
query.exec("Select ProductName from "+ table);
 
//routine to read values
while (query.next()) {
 
QSqlRecord record = query.record();
 
//get ProductNames
ProductNames << query.value(record.indexOf("ProductName")).toString();
numberOfProducts++;
}

//check for errors on database connection
if (!db.open()) {
qDebug() << db.lastError();
         // return 4;
}
//set table to use
QString table = "mytable";
QSqlQuery query("USE " + table);

//Query to get table info
query.exec("Select ProductName from "+ table);

//routine to read values
while (query.next()) {

QSqlRecord record = query.record();

//get ProductNames
ProductNames << query.value(record.indexOf("ProductName")).toString();
numberOfProducts++;
}

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How should I close it now? I already tryed, db.close(), query.clear, db.removeDatabase("QMYSQL"), delete db, delete query, QSqlDatabase::removeDatabase("QMYSQL"), etc...
Remind you that the application works most of the times but the message keeps on comming.

[Lykurg]

Simple solution: Don't use a member variable for QSqlDatabase! There is no need for it. You can always use the static QSqlDatabase::database() function to get a handle when you need it.

[ruben.rodrigues]

do you mean that I don't need the db variable?
because I have the QSqlDatabase::database() but there is no function to set the username, password, database name and host..

or am I confused?

[Lykurg]

You need it, but not as a member variable:

//somewhere in your definition
void Foo::bar()
{
  //...
  {
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL"); //select database type
    db.setHostName("localhost"); //tell where the database is
    db.setDatabaseName("qt");    //Database name
    db.setUserName("user");       //username to connect to the database
    db.setPassword("password");  //password from user
  } // here the temporary "db" gets deleted.
  //...
}
 
void Foo::baz()
{
  QSqlQuery q(QSqlDatabase::database());
  // use q
} // here q gets deleted...

//somewhere in your definition
void Foo::bar()
{
  //...
  {
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL"); //select database type
    db.setHostName("localhost"); //tell where the database is
    db.setDatabaseName("qt");    //Database name
    db.setUserName("user");       //username to connect to the database
    db.setPassword("password");  //password from user
  } // here the temporary "db" gets deleted.
  //...
}

void Foo::baz()
{
  QSqlQuery q(QSqlDatabase::database());
  // use q
} // here q gets deleted...

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