autocomplete and database updates

I'm trying to figure out a way to have the rails3-auto-complete plugin
pull back a Name field in the text box but when saving to the database,
that it would only save a hidden id field and not the name (for database
normalization purposes). Here is my example

Category
id
name

Item
id
name
description
category_id

When I do the text field for autocomplete
<input type="text" name="item[category]" value="TestValue" />
<input type="hidden" name="category_id" value="2" />

I would only want the hidden value submitted and I don't know what a
good approach or recommendation woudl be. I could denormalize the
database but I don't want too and I could also put some of these
categories into just a standard file and read from them...but would like
to learn how to do this properly within Rails.