Hey Susi!, Hope this will work for u.. 

samp_cal1=fields.Integer(string="a")

samp_cal2=fields.Integer(string="b")

samp_cal3=fields.Integer(string="c",readonly=True)

@api.onchange('samp_cal1','samp_cal2')

def adding_samp(self):

    self.samp_cal3=self.samp_cal1+self.samp_cal2

@api.model

def create(self,vals):

    if self.samp_cal3:

        vals['samp_cal3']=self.samp_cal1+self.samp_cal2

    res = super(tokenreservation, self).create(vals)

    return res

@api.multi

def write(self,vals):

    if not self.samp_cal3:

        vals['samp_cal3'] = self.samp_cal1+self.samp_cal2

    res = super(tokenreservation, self).write(vals)

    return res

assighn values like this

cr.execute("UPDATE  hr_analytic_timesheet SET approver = %s where id = %s ", (  your value, id of object ))

try to write the value into the field using SQL query

You can update a many2one field, with either an ID or record.

You can’t put any ID or record, a check is made on the relationship to ensure data integrity

 You have to set the value of this field in the create method explicitly

def create(self, cr, uid, val, context=None):
...
    val['approve'] = xyz_id # this is the user id you decided that he will do the approval
    res = super(you_class, self).create(cr, uid, val, context)
    return res