function based index as a conditional unique key? I have a search table which has a generated primary key PK_ID,and also has primary keys from three or four other tables in the FK_ID column  

CREATE TABLE METADATA  
(  
  PK_ID        NUMBER(10)                       NOT NULL,  
  OBJ_OWNER    VARCHAR2(35 BYTE)                NOT NULL,  
  OBJ_PACKAGE  VARCHAR2(35 BYTE),  
  OBJ_NAME     VARCHAR2(35 BYTE)                NOT NULL,  
  OBJ_TYPE     NUMBER(10)                       NOT NULL,  
  FK_ID        NUMBER(10)                       NOT NULL  
);


For example, when OBJ_TYPE = 1 then every FK_ID should be unique for that value of OBJ_TYPE.

I think I can do this with a function based index but the link to Tom Kyte's discussion on this is broken across a few web sites and I'm not sure how.
(Oracle 9.2.0.8)

A composite index will do the job.  A bonus would be if  a function based index could be used as a foreign key for the other tables.  Is this possible?
I'm not completely sure that I understand what you mean by "every FK_ID should be unique for that value of OBJ_TYPE". 

If you are trying to say that each FK_ID has to be unique within each OBJ_TYPE, then it sounds like you just want a composite index on (OBJ_TYPE, FK_ID). 

CREATE UNIQUE INDEX idx_uniq_metadata
   ON metadata( obj_type, fk_id );


If, on the other hand, you are trying to say that if OBJ_TYPE = 1 then FK_ID must be unique but you can have duplicate FK_ID values if the OBJ_TYPE is something else, then you'd want a function-based unique index.

CREATE UNIQUE INDEX idx_uniq_metadata2
   ON metadata( (case when obj_type = 1 
                      then fk_id 
                      else null 
                   end) );