Field.function / method not automatically updating field

i have a function which currently works when a user pick and item from  a dropdown menu it changes the value of  another field.

however the other field only changes when i click save how can i make this that as soon as i change the value of  the dropdown field the , other fields value changes without having to click save.

.py file

class account_analytic_account(osv.osv):

_name = "account.analytic.account"

_inherit = "account.analytic.account"

def get_rental(self,cr,uid,ids,context,table1,obj2):

x={}

obj1 = self.pool.get('mm.control')

for record in self.browse(cr, uid , ids):

table1 = obj1.search(cr,uid,[('company','=',record.invcompany_id.id)])

obj2=obj1.browse(cr,uid,table1[0])

x[record.id]=obj2.rental_split

print table1

print record.invcompany_id

print obj2.rental_split

return x

_columns = {

'invcompany_id': fields.many2one('res.company', 'Invoicing Company ' , domain="[('company_type', '=', 'Property Company')]" ,required=True, select=1),

'rental_split': fields.function(get_rental , method=True ,string= 'Rent', store=False, type="integer", help="Rent split of total rent amount *25"),

}

.xml

<group>

<field name="rental_split"/>

<field name="invcompany_id"/>

</group>

Any ideas to make the field automatically update without saving