Observe_field for two fields

Rails
1

Hi

I have now three select boxes(This is my current structure of
program)

Priority <%= select( "sd_ticket", "sd_priority_id", SdPriority.find(:all).collect {|p| [ p.name, p.id ] }) %> 
<th width="%"><label for="" >Urgency</label></th>
<td width=""><%= select( "sd_ticket", "sd_urgency_id",

SdUrgency.find(:all).collect {|p| [ p.name, p.id ] }) %>

Impact <%= select( "sd_ticket", "sd_impact_id", SdImpact.find(:all).collect {|p| [ p.name, p.id ] }) %>

I have to make a change to the above as, based on what value I choose
for Urgency and Impact the value in Priority is to be changed…So
Priority is to be replaced by a label box .The table structure as
below…How can I do this with observe_field.There is a table for
sd_prioritizations.So more clearly The impact and urgency are first read
from the sd_impact and sd_urgency tables.And when user changes the two
combination of that it is first checked in sd_prioritization to get
Priority and the corresponding priority name is read from sd_priority
table to get Priority

1)sd_impacts
 id  integer not null
 name  character varying(80)

2)sd_urgencies
  id  integer not null
 name  character varying(80)

3)sd_priorities
  id  integer not null
 name  character varying(80)
4)sd_prioritizations
 id integer
 service_desk_impact_id   integer
 service_desk_urgency_id  integer
 service_desk_priority_id   integer

Sijo