Why delete_all works but not delele(nn)?

I am able to delete all the comments with:

However I am unable to delete a row in the table...

I've tried several things but as far as I know from some examples
should work.

I'm I doing something very wrong?

Thanks once again.

Actually @tab.comments will return an array.
"delete" method is available for ruby Array class.

But there is no method called "delete_all" for Array class.

Actually it returns an AssociationProxy that's pretending to be an