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