restrict lov depend on another partial lov

  i have 4 segmentscompany

department

building

room

department,building and room depend on company

so no relation between building and room .

build_vs 10 soap

              20 plastic

room_vs 1001 room1

               1002 room2

                2001 section1

i want when add asset when select build_vs 10 the lov of room_vs appears only room1 and room2(because i select 10 but in room_vs i substr the first 2 digits and appear the other digits)