class SalesmanCommission(models.Model):
   _name = 'salesman.commission'
   _rec_name = 'partner_id'

   @api.depends('commission_line.commission')
   def _compute_commission(self):
      for rec in self:
         # return '250'
         rec.update({'commission': sum(rec.commission_line.mapped('commission'))})
         print self.commission

   partner_id = fields.Many2one('res.users', string='SalesMan')
   commission_line = fields.One2many('salesman.commission.line', 'line_id', 'Commission Line')
   commission = fields.Monetary(compute='_compute_commission', string='Commission', readonly=True)
   currency_id = fields.Many2one('res.currency', default=lambda self: self.env.user.company_id.currency_id.id)



class SalesmancommissionLine(models.Model):
   _name = 'salesman.commission.line'

   order_id = fields.Many2one('sale.order', 'Order')
   commission = fields.Monetary('Commission')
   currency_id = fields.Many2one('res.currency', default=lambda self: self.env.user.company_id.currency_id.id)
   date = fields.Date('Date')
   line_id = fields.Many2one('salesman.commission', string='Line Id')

<record model="ir.ui.view" id="view_salesman_commission_pivot">
    <field name="name">salesman.commission.pivot</field>
    <field name="model">salesman.commission</field>
    <field name="arch" type="xml">
        <pivot string="Salesman Commission Line">
            <field name="commission" type="measure"/>
        </pivot>
    </field>

</record> 

while in sale module amount_total  is also compute but it is working in pivot 

can anyone help, thanks.