how to handle collection_select

i have already used 'collection_select' select helper for drop down box.
it works properly and retrieve data from database(mysql).Bt I did't not
select any
particular data from database and did't fire the quary

for drop down I use
this syntax
<%= collection_select(:practices, :id, @cfohomes, :id,
:cfod_suniversity, options ={:prompt => "-Select a University"}, :class
=>"practices") %>

here cfod_suniversity is a attribute of my database

plz provide a simple code to insert the selected item into database