using model to fill tableView with QMYSQL

JonB

@Thank-You
But you have an ongoing ui->information->setModel(model) which is still using the database, and you wonder why it goes wrong after you have done a removeDatabase()? Where does it say that situation is a good idea?

jsulm

@Thank-You said in using model to fill tableView with QMYSQL:

QSqlDatabasePrivate::addDatabase: duplicate connection name 'report', old connection removed.

Simply add the database once...

Thank You

@jsulm
https://doc.qt.io/qt-5/qsqldatabase.html#removeDatabase
Warning: There should be no open queries on the database connection when this function is called, otherwise a resource leak will occur.

{
    QSqlDatabase db = QSqlDatabase::database("sales");
    QSqlQuery query("SELECT NAME, DOB FROM EMPLOYEES", db);
}
// Both "db" and "query" are destroyed because they are out of scope
QSqlDatabase::removeDatabase("sales"); // correct

From the official documentation I found that queries should not be open.
I don't know wether I am understanding what you are saying or not. If you mean different thing, please describe in simpler words

jsulm

@Thank-You I was talking about ADDING database.
"QSqlDatabasePrivate::addDatabase: duplicate connection name 'report', old connection removed." - this means you're trying to add a connection which already was added...

What you posted now shows again that you still are removing the database though you was told several times already to not to do so. Why?!

JonB

@Thank-You said in using model to fill tableView with QMYSQL:

From the official documentation I found that queries should not be open.

Again: in your case the query is not out of scope/destroyed because you have:

        QSqlQueryModel *model = new QSqlQueryModel();
        QSqlQuery *query = new QSqlQuery(db);
        model->setQuery(*query);
        ui->information->setModel(model)

Thank You

@jsulm
If I am right ,
We get this error when we didn't close the database properly.

Please see my code at very top ,
There I created database and after executing it
I just removed the database

Yes as said by @Christian-Ehrlicher and @VRonin If I remove this line it actually works but this error is obtained.

Again I will tell you ,
It was working fine when I used the "SQLITE" database, The same code. Even there is no fault in query too.
What is difference between using these database, Is it due to ASYNCHRONUS thing?
Am I right ??

JonB

@Thank-You
We have tried to tell you. And one more time: just because bad code might work with SQLite but behave differently with MySQL does not mean we can tell you why you might get away with it in some circumstances.

Why don't you just think about what you are doing? If you add database, open a query onto it and then set a permanent model to that which is referenced by an outside-world QTableView, and then you remove the database while that is still the case, what do you actually expect to happen? Can you not see this might be a problem?

Don't remove the database while you have a view/model using it.
Don't re-add the same database while you have previously added it and not yet removed it.

Frankly I don't understand your whole architecture of opening a database while you are displaying a message box, and leaving it to persist into the outside world. The usual approach is to open a database initially/first time wanted and persist it till you are ready to close it and never want it again. I wouldn't be adding/removing the same database repeatedly.

Thank You

Don't remove the database while you have a view/model using it

How can I know that it is using the database or not??

I just tried inserting QMessageBox when

if(query->exec()){
// same as the upper code here 
QMessageBox::information(this,"NOw the database is shown", " This is also shown");
}

After clicking OK,

The data vanishes again
@jsulm @JonB can you see this?

JonB

@Thank-You
After showing the message box, does your code execute QSqlDatabase::removeDatabase()? Just a "yes" or "no" answer.

Thank You

@JonB
No it doesn't
It executes
removeDatabase
at the very end of the program
like

{
db initialize
execute query and set model
show the message box
}
remove the database

or you can see the code same but there is
messagebox inside if(query->exec){
// all execution like model etc
messagebox
}

JonB

@Thank-You
If you really want anyone to look at this, show your actual relevant, minimal code which reproduces the issue, not a description of it.

Thank You

@JonB I have posted the code right at the top
Exactly that code brings this error

void tableFill::fill_table(){
QString command = "select * from countryDetails;";
   {
QString databaseName = "root";
QString databasePassword= "";
QString databaseUsername= "root";
QString databaseHost = "localhost";
int databaseName = 3306;


       QSqlDatabase db =  QSqlDatabase::addDatabase("QMYSQL");
       db.setDatabaseName(databaseName);
db.setPort(databasePort);
db.setHostName(databaseHostname);
db.setUserName(databaseUsername);
db.setPassword(databasePassword);
    
       if(db.open()){
           QSqlQueryModel *model = new QSqlQueryModel();
           QSqlQuery *query = new QSqlQuery(db);
           query->prepare(command);
           if(query->exec()){
               model->setQuery(*query);
               ui->information->setModel(model);
//setting the model 
           }else{
               QMessageBox::warning(this,"ERrors in query exec" ,"Error while using query execution"+query->lastError().text());
           }

       }else{
           QMessageBox::warning(this,"Error in database ", "Database is not connected "+db.lastError().text());
       }

   }
   QSqlDatabase::removeDatabase("qt_sql_default_connection"); 
}

Christian Ehrlicher

@Thank-You And we told you at least two times that you must not remove the database connection while there is an open query for your QSqlTableModel!

JonB

@Thank-You
This is exactly the code as it was. It has removeDatabase() immediately after the message box show. Which we have told you multiple times is wrong. It does not have the

QMessageBox::information(this,"NOw the database is shown", " This is also shown");

you later claimed to have put in, nor the output you showed in your last screenshot. Which doesn't correspond to that either.

What else is there to say? You don't listen, you don't adjust your code, you show screenshots which don't pertain to the code.

I think we are all done. We don't know what answer you are prepared to accept. I suggest you keep your code exactly as you have it, since that is the only thing you are prepared to accept. Bye.

Thank You

@Christian-Ehrlicher And when to remove it
Or leave it without removing ?
I said that it will give me errors,
QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.
I don't want this application to have this error?

OR
How can I get the signal when query does not require the database.

@JonB Please refer to my last code. It is exactly the same, Nothing different. May be I gave you the example of another table fill.
Just see the last one and say exactly what is the problem just for the last time.

jsulm

@Thank-You said in using model to fill tableView with QMYSQL:

I said that it will give me errors,

And I said you to not to add the database again and again, do it just once. I really don't know why it is necessary to repeat same things again and again...
Remove the database when it is not needed anymore (for example in the destructor of your class).

Thank You

@jsulm OOOOw
Maybe it can help ,
I will try and say the result.

Nekreg45

Maybe an analogy would help with the advice that everybody has given. Pretend like running a query on a database is like teaching a dog a new trick. You want the dog to be alive when you teach the trick and still be alive to enjoy watching the trick.

What people are saying is that in your fill table function, you are creating a dog, teaching a trick, killing the dog, and wondering how come it doesn't perform the trick.

In conclusion, you need your dog to live as long as the lifetime that you want the user to see your table. Did this help?