Three way foreign keys

Hi all,

I'm implementing just-in-time printer consumable ordering within my inventory 
system and utilising SNMP printer interrogation. That bit seems fairly 
straight forward.

The bit I'm stuck with is the schema, which I know I've done before by my 
brain isn't functioning today.

I have the following tables

hw_types        - printer make / model
consumables    - printer consumables
type_consumables    - n-to-n relationship
pieces        - equipment of type in hw_types

I need to create a levels record for each consumables <-> pieces pair.

p_id from pieces
cs_id from consumables

In other words the foreign key constraint needs to look at the type field 
(p_type) in the pieces field and check for the record (p_type / cs_id) 
existing within the type_consumables

Hope that's clear enough. Below are the (simplified) table definitions.

users=# \d hw_types                                    Table "public.hw_types"   Column    |         Type          |
                    Modifiers                         
 
--------------+-----------------------+-----------------------------------------------------------hwt_id       |
integer              | not null default 
 
nextval('hw_types_hwt_id_seq'::regclass)
Indexes:   "hw_types_pkey" PRIMARY KEY, btree (hwt_id)
Foreign-key constraints:   "hw_types_hwt_cat_fkey" FOREIGN KEY (hwt_cat) REFERENCES 
hw_categories(hwc_id)   "hw_types_hwt_replaced_fkey" FOREIGN KEY (hwt_replaced) REFERENCES 
hw_types(hwt_id)

users=# \d consumables                                   Table "public.consumables"  Column    |         Type
|                         Modifiers                          
 
-------------+-----------------------+-------------------------------------------------------------cs_id       |
integer              | not null default 
 
nextval('consumables_cs_id_seq'::regclass)
Indexes:   "consumables_pkey" PRIMARY KEY, btree (cs_id)
Foreign-key constraints:   "consumables_cs_type_fkey" FOREIGN KEY (cs_type) REFERENCES 
cons_types(cst_id)

users=# \d type_consumables
Table "public.type_consumables"Column |  Type   | Modifiers 
--------+---------+-----------hwt_id | integer | not nullcs_id  | integer | not null
Indexes:   "type_consumables_unique_index" UNIQUE, btree (hwt_id, cs_id)
Foreign-key constraints:   "type_consumables_cs_id_fkey" FOREIGN KEY (cs_id) REFERENCES 
consumables(cs_id)   "type_consumables_hwt_id_fkey" FOREIGN KEY (hwt_id) REFERENCES 
hw_types(hwt_id)

users=# \d pieces                                         Table "public.pieces"     Column       |            Type
      |                       
 
Modifiers                       
-------------------+-----------------------------+-------------------------------------------------------p_id
  | integer                     | not null default 
 
nextval('pieces_p_id_seq'::regclass)p_type            | integer                     | not null
Indexes:   "pieces_pkey" PRIMARY KEY, btree (p_id)
Foreign-key constraints:   "pieces_p_type_fkey" FOREIGN KEY (p_type) REFERENCES hw_types(hwt_id)

users=# 
-- 
Gary Stainburn
Group I.T. Manager
Ringways Garages
http://www.ringways.co.uk