After days of struggling I ve managed to solve the issue. It was a stupid mistake I had done in the code shown above and unfortunately quite difficult to detect since I was not getting any kind of error message, except from the INSERT query failing. I am posting the solution below:

The problem was caused by line 65 of the code:
Qt Code: 
  1. query.bindValue(":screw_type_id",(int)current_tooth->getScrewTypeID()>0? current_tooth->getScrewTypeID():0);
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and it is related to the column
Qt Code: 
  1. [int(11)]screws_idscrews,
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of the SQL table. This column is a foreign key to another table. Therefore, it is automatically constraint not to take any values that will point to non-existent entries in the other table. As a result, the conditional statement above failed when it returned 0, since the foreign key cannot take the value 0 as it was setup. The correct value that the statement should return is NULL. To do this, line 65 was changed to this:
Qt Code: 
  1. query.bindValue(":screw_type_id",current_tooth->getScrewTypeID()>0? current_tooth->getScrewTypeID():QVariant(QVariant::Int));
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This solved the problem.

Simply using NULL as the 2nd argument to QSqlQuery::bindValue() does not work. According to the documentation of QSqlQuery::bindValue()
To bind a NULL value, use a null QVariant
.
Here, the column screws_idscrews is of type INT so the following must be used to get a NULL INT:
Qt Code:
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