I'm using web2py to create an application

I've got 2 different roles: client and referents. Every client can have multiple referents. 
Both client and referent can insert row in a table, called object.

The user table, where client and referent are registered, has a column called "client_id".
Every referent in the column_id has the id of the client to whom he reference to.

The problem is: which value should a client have in client_id?
A possible answer could be NULL or the id of the client itself.

Which one is better? Or there's something better?


Question: How do I automate the handling of a problem (no network device found) in Ubuntu 10.04 w/ preseed?
Answer: I think this is what you are looking for:
d-i netcfg/enable boolean false. I found it here.