there is no solution without writing in database ? the information is in database, can't I read it directly ?

Fields related are not stored in DB by default unless parameter store=True is not defined.

thanks a lot

another question: how can I set the partner_id field with the project.partner_id field by default ?

You need to use default_get method and update it's result with with value that you need. More reference on http://doc.openerp.com/v6.0/developer/2_5_Objects_Fields_Methods/methods.html

thanks but I don't understand how to use this method. thanks again