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...

On an association collection like you've got, delete_all deletes all
the rows in the collection, but delete deletes the object you pass in
- it doesn't operate on ids.
If you want to delete a comment by id, you need to use the delete
method on the class (ie Comment.delete 22