I try to show supplier name in product tree view so i create this function :

class product_product(osv.osv):

    _inherit = 'product.product'

    def get_supplier_name(self, cr, uid, *args):
        cr.execute("select pt.name from res_partner pt  join product_supplierinfo fr on (fr.name=pt.id)  where fr.sequence = 1")
        res = cr.fetchone()
        return res

    _columns = {
      'supplier_name': fields.function(get_supplier_name, method=True,string="Supplier name",type="many2one", store = True),
      }


product_product()

But I have this error :

    Traceback (most recent call last): File "netsvc.pyo", line 489, in dispatch File "service\web_services.pyo", line 599, in dispatch File "osv\osv.pyo", line 122, in wrapper File "osv\osv.pyo", line 176, in execute File "osv\osv.pyo", line 167, in execute_cr File "osv\orm.pyo", line 2940, in read File "osv\orm.pyo", line 3060, in _read_flat File "osv\fields.pyo", line 799, 
in get AttributeError: 'tuple' object has no attribute 'values'

What can I do?