comparing selection of a drop down using ajax

I am very new to ruby and since my javscript skills are zero I was
wondering if this could be done.

I have a drop down. When the user makes a selection I would like to
make a call with observe_field.

Can I then check to see if the selection the user made from the drop
down menu is in a another array that keep track of what the user has
selected.

In the array that is checked against will simply add 1 next to the
item if it is not in the list. That is the user is selecting it for
the first time.

My English is not very good so I hope this makes sense.

Can someone give me some guidance on this.

yeah you can get the value of what the select is set at and add it to
a javascript variable or what not. Google for select javascript.

So I used <%= observe_field :id_of_select, url => {action =>
"add"}, :frequency => 0.25 %>

this is below the select tag when I choose and item from the drop down
it does absolutely nothing.
I examined it in firebug and I don't see anything either.

View of the source shows that it has selected the proper id of the
drop down.

I got something to happen in firebug, but it is calling function 3 or
4 times while the user goes down the drop down menu.
Also I can not get it do anything with .js file no out come , tried a
simple page.insert.

Is observe_field the correct method?

I have map.connect ':controller/:action/:id.:format'
below it I have a catch all map.connect '*path', :controller =>
'user', :action => 'login'

I'm not sure how to do rjs with observe_field.