Adding validation to text fields with autocomplete and to select fields

Hello everybody,

I’m just learning rails right now and for simple CRUD operations I’m
using
scaffold to generate some code. I’m using Rails 1.2.6.
I’ve changed some of the fields to use an autocomplete feature and
validation using validate_presence_of is not working on these fields.
The
problem is that this fields are related to stuff inside a different
model,
like:

I have two tables, one called artist and the other called album. Album
has a
foreign key to Artist. So in the _form.rhtml for the album i have a
field
with autocompletion for the name of the artist. In the album controller
I
make a search for the artist with the received name and associate it to
the
newly created instante of Album. But if the user sends the form with the
artist_name empty, I don’t know how to use the validation helpers…

This isn’t working for select fields nether. Putting it simple, it just
works for ordinary text fields.

Anybody?

Thank you very much!

Cássio Marques