I like the idea but then a simple bool might not be enough as a return value, because we have at least 2 ways this can fail sqlerror,directory not existend/readible.

I would go with that this function needs a QDir and then only returns true/false on sql error and have the existence check done by the caller. We do that already now in a way because the caller checks if the string is empty. Also then the DAO's are only responsible for the sql and nothing else