Hello,

To support a forum application I have a "forum" table:

   Column    |            Type             
-------------+-----------------------------
 created_by  | integer                     
 created_on  | timestamp without time zone 
 modified_by | integer                     
 modified_on | timestamp without time zone 
 id_forum    | integer                     
 id_parent   | integer                     
 subject     | text                        
 message     | text                        
 id_show     | integer                     
 id_thread   | integer                     
 id_person   | integer                     


The id_show and id_person columns are meant to point to a show or person 
object about which the specific forum thread is started (threads can 
also have a "free" subject and these fields are then empty). 

Now I'd like to add forums about locations (theatres, museums, etc.) so 
I am thinking about adding an id_location column, but this method seems 
a bit inelegant to me: 

- every subsequent message of the thread will carry these useless fields 
  (only the first message in thread uses one of them),

- if I want to connect a forum to a new type of object, yet a new 
  id_<object> will be required,

Is there a better way to have a single field that can relate with 
several objects and still benefit from integrity checks?

Thanks,

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