[web2py] How do I do complex validation?

[Jason Brower]

Is there a way in the model file to set a bit more complex set-ups?  It 
is in the book?
For examples I have:
user_role.id 
<http://127.0.0.1:8000/InterestID3/appadmin/select/db?orderby=user_role.id> 
	user_role.user_id 
<http://127.0.0.1:8000/InterestID3/appadmin/select/db?orderby=user_role.user_id> 
	user_role.role_id 
<http://127.0.0.1:8000/InterestID3/appadmin/select/db?orderby=user_role.role_id> 
	user_role.conference_id 
<http://127.0.0.1:8000/InterestID3/appadmin/select/db?orderby=user_role.conference_id> 
	user_role.checked_in 
<http://127.0.0.1:8000/InterestID3/appadmin/select/db?orderby=user_role.checked_in> 


and I want to make sure that there is no user_id and conference_id 
duplicatants.
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Best Regards,
Jason Brower