drop down list from a scaffold

I have a largely-default scaffold at <http://strawr.googlecode.com/svn/
trunk/app/views/feeds/show.rhtml> to which I'd like to add a drop down
list.

The join table between "categories" and "feeds" is "tags" (for want of a
better name). It's an association of the "has many through" variety.
From the console It's quite easy to create a row in "feeds", create a row
in "categories" and then to create a row in "feeds" which links the
"feeds" row with the "categories" row. It seems to work find as expected
from the console: a many-to-many relationship.

To create a drop down list of categories from <http://localhost:3000/
feeds/show/1> is what I'm after. Right now it's text box which I'd like
to change to a drop down list populated with all of the rows from
"categories".

thanks,

Thufir