you can use cursor(or cr) present in a method..

almost all ORM methods will have these paramaters: self, cr, uid

so one can use cr, which is nothing but cursor paramater used to deal with database...

Example: cr.execute("select * from res_partner")

And to do validation: you can set _sql_constraints in your object
 

Hint: use Ctrl + H to search in you the addons, if you are using EDI like eclipse/aptana studio