when inherit sale order line, onchange('product_id') can't work, but onchange other fields can work normally. 

also find that when inherit sale order , onchange('partner_id') can't work  also. 

class SaleOrderLine(models.Model):

_inherit = "sale.order.line"

@api.one

@api.onchange('delay')

def onchange_commission_plan(self):

print '323a'

if self.product_id:

pass

@api.one

@api.onchange('product_id')

def onchange_commission_plan99(self):

print '121323a'

if self.product_id:

pass