Fetching data from table view

[JD2000]

How does

lsn.deleteLesson(ui->tableView_lessons->currentIndex())

lsn.deleteLesson(ui->tableView_lessons->currentIndex())

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In your button clicked calling function relate to

void lessons::deleteLesson(QModelIndex index)
{
QSqlTableModel *model = this->lessonsTable()
.....
}

void lessons::deleteLesson(QModelIndex index)
{
QSqlTableModel *model = this->lessonsTable()
.....
}

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You are passing a tableview index in the calling function to who knows what in the deleteLesson slot.

Also have you connected the signal and slot properly?