Understanding the Issue with Saving Data in a Qt Application

In this article, we’ll delve into the world of Qt programming and explore why data inserted into a database in a Qt application seems to be lost after the application is closed and reopened.

Background

Qt is a cross-platform application development framework that provides a comprehensive set of libraries and tools for building GUI applications. One of its key features is support for various databases, including SQLite. In this article, we’ll focus on understanding why data inserted into a SQLite database using Qt seems to be lost after the application is closed.

Connection to the Database

Before we dive into the details, let’s take a look at how the connection to the database is established in the dbliteratura() function:

inline void dbliteratura(){
    QString sqlfile = "../dbliteratura.sql";
    QFile   f(sqlfile);
    if (!f.open(QIODevice::ReadOnly))
        {
            QMessageBox::critical(0, "IO Error", "Couldn't open file" + sqlfile + " for reading");
            exit(1);
        }

    QSqlDatabase db = QSqlDatabase::database();
    if (!db.isOpen())
        {
            QMessageBox::critical(0, "Database Error", "Database is not open");
            exit(2);
        }

    // ...
}

In this code snippet, we’re opening a file named dbliteratura.sql and reading its contents into the sql variable. The sql variable is then used to execute queries on the database.

Executing Queries

Let’s take a closer look at how the queries are executed:

QSqlQuery q;
char z;

db.transaction();
while ( f.getChar(&amp;z) )
    {
        sql = z;
        while (f.getChar(&amp;z) &amp;&amp; (z != ';'))  sql.push_back(z);
        q.exec(sql);
    }

In this code snippet, we’re using a QSqlQuery object to execute queries on the database. The query is constructed by reading the SQL file character by character and appending each character to the sql variable until we reach a semicolon (;). Once we’ve reached the semicolon, we execute the query using the exec() method.

Committing Changes

After executing all the queries in the SQL file, we’re committing the changes:

db.commit();

This is done to ensure that any changes made to the database are persisted.

Conclusion

In this article, we’ve explored why data inserted into a SQLite database using Qt seems to be lost after the application is closed. We’ve taken a closer look at how the connection to the database is established and how queries are executed. While our code snippet may seem correct, there’s an important consideration that we’re overlooking.

In our dbliteratura() function, we’re executing each query on its own using the exec() method. However, this approach has a limitation: it doesn’t allow us to batch multiple queries together into a single transaction. This can lead to issues when dealing with complex database operations.

To avoid these issues, you should consider using a different approach to execute queries, such as executing them in batches or using transactions to group related queries together.

Next Steps

In the next article, we’ll explore an improved approach for executing queries on a SQLite database using Qt. We’ll discuss how to use transactions to batch multiple queries together and improve performance.

Improving Query Execution Using Transactions

Let’s take a closer look at how we can modify our dbliteratura() function to use transactions:

inline void dbliteratura(){
    QString sqlfile = "../dbliteratura.sql";
    QFile   f(sqlfile);
    if (!f.open(QIODevice::ReadOnly))
        {
            QMessageBox::critical(0, "IO Error", "Couldn't open file" + sqlfile + " for reading");
            exit(1);
        }

    QSqlDatabase db = QSqlDatabase::database();
    if (!db.isOpen())
        {
            QMessageBox::critical(0, "Database Error", "Database is not open");
            exit(2);
        }

    QTransaction transaction(db);

    while ( f.getChar(&amp;z) )
        {
            sql = z;
            while (f.getChar(&amp;z) &amp;&amp; (z != ';'))  sql.push_back(z);
            QString query = "INSERT INTO table_name (" + sql + ");";
            transaction.exec(query);
        }

    transaction.commit();
}

In this modified version, we’re using a QTransaction object to group related queries together. The transaction() function is used to start the transaction, and the commit() function is used to commit the changes.

By using transactions, we can avoid issues that arise when executing multiple queries on their own using the exec() method.

Conclusion

In this article, we’ve explored why data inserted into a SQLite database using Qt seems to be lost after the application is closed. We’ve taken a closer look at how the connection to the database is established and how queries are executed. By improving our approach for executing queries using transactions, we can improve performance and avoid issues that arise when dealing with complex database operations.

Final Thoughts

To summarize, data inserted into a SQLite database using Qt seems to be lost after the application is closed because of how queries are executed. By using transactions to group related queries together, we can avoid these issues and improve performance.

Last modified on 2024-12-03