Hi,

The onchange method product_id_change is defined in sale module in sale.py file's sale.order.line model.

def product_id_change(self, cr, uid, ids, pricelist, product, qty=0,
            uom=False, qty_uos=0, uos=False, name='', partner_id=False,
            lang=False, update_tax=True, date_order=False, packaging=False, fiscal_position=False, flag=False):
        if not  partner_id:
            raise osv.except_osv(_('No Customer Defined !'), _('You have to select a customer in the sales form !\nPlease set one customer before choosing a product.'))
        warning = {}
        product_uom_obj = self.pool.get('product.uom')
        partner_obj = self.pool.get('res.partner')
        product_obj = self.pool.get('product.product')
        if partner_id:
            lang = partner_obj.browse(cr, uid, partner_id).lang
        context = {'lang': lang, 'partner_id': partner_id}

        if not product:
            return {'value': {'th_weight': 0, 'product_packaging': False,
                'product_uos_qty': qty, 'tax_id':[]}, 'domain': {'product_uom': [],
                   'product_uos': []}}
        if not date_order:
            date_order = time.strftime('%Y-%m-%d')

        result = {}
        product_obj = product_obj.browse(cr, uid, product, context=context)
        if not packaging and product_obj.packaging:
            packaging = product_obj.packaging[0].id
            result['product_packaging'] = packaging

        if packaging:
            default_uom = product_obj.uom_id and product_obj.uom_id.id
            pack = self.pool.get('product.packaging').browse(cr, uid, packaging, context=context)
            q = product_uom_obj._compute_qty(cr, uid, uom, pack.qty, default_uom)
#            qty = qty - qty % q + q
            if qty and (q and not (qty % q) == 0):
                ean = pack.ean or _('(n/a)')
                qty_pack = pack.qty
                type_ul = pack.ul
                warn_msg = _("You selected a quantity of %d Units.\n"
                            "But it's not compatible with the selected packaging.\n"
                            "Here is a proposition of quantities according to the packaging:\n\n"
                            "EAN: %s Quantity: %s Type of ul: %s") % \
                                (qty, ean, qty_pack, type_ul.name)
                warning = {
                    'title': _('Picking Information !'),
                    'message': warn_msg
                    }
            result['product_uom_qty'] = qty

        uom2 = False
        if uom:
            uom2 = product_uom_obj.browse(cr, uid, uom)
            if product_obj.uom_id.category_id.id != uom2.category_id.id:
                uom = False
        if uos:
            if product_obj.uos_id:
                uos2 = product_uom_obj.browse(cr, uid, uos)
                if product_obj.uos_id.category_id.id != uos2.category_id.id:
                    uos = False
            else:
                uos = False
        if product_obj.description_sale:
            result['notes'] = product_obj.description_sale
        fpos = fiscal_position and self.pool.get('account.fiscal.position').browse(cr, uid, fiscal_position) or False
        if update_tax: #The quantity only have changed
            result['delay'] = (product_obj.sale_delay or 0.0)
            result['tax_id'] = self.pool.get('account.fiscal.position').map_tax(cr, uid, fpos, product_obj.taxes_id)
            result.update({'type': product_obj.procure_method})

        if not flag:
            result['name'] = self.pool.get('product.product').name_get(cr, uid, [product_obj.id], context=context)[0][1]
        domain = {}
        if (not uom) and (not uos):
            result['product_uom'] = product_obj.uom_id.id
            if product_obj.uos_id:
                result['product_uos'] = product_obj.uos_id.id
                result['product_uos_qty'] = qty * product_obj.uos_coeff
                uos_category_id = product_obj.uos_id.category_id.id
            else:
                result['product_uos'] = False
                result['product_uos_qty'] = qty
                uos_category_id = False
            result['th_weight'] = qty * product_obj.weight
            domain = {'product_uom':
                        [('category_id', '=', product_obj.uom_id.category_id.id)],
                        'product_uos':
                        [('category_id', '=', uos_category_id)]}

        elif uos and not uom: # only happens if uom is False
            result['product_uom'] = product_obj.uom_id and product_obj.uom_id.id
            result['product_uom_qty'] = qty_uos / product_obj.uos_coeff
            result['th_weight'] = result['product_uom_qty'] * product_obj.weight
        elif uom: # whether uos is set or not
            default_uom = product_obj.uom_id and product_obj.uom_id.id
            q = product_uom_obj._compute_qty(cr, uid, uom, qty, default_uom)
            if product_obj.uos_id:
                result['product_uos'] = product_obj.uos_id.id
                result['product_uos_qty'] = qty * product_obj.uos_coeff
            else:
                result['product_uos'] = False
                result['product_uos_qty'] = qty
            result['th_weight'] = q * product_obj.weight        # Round the quantity up

        if not uom2:
            uom2 = product_obj.uom_id
        if (product_obj.type=='product') and (product_obj.virtual_available * uom2.factor < qty * product_obj.uom_id.factor) \
          and (product_obj.procure_method=='make_to_stock'):
            warning = {
                'title': _('Not enough stock !'),
                'message': _('You plan to sell %.2f %s but you only have %.2f %s available !\nThe real stock is %.2f %s. (without reservations)') %
                    (qty, uom2 and uom2.name or product_obj.uom_id.name,
                     max(0,product_obj.virtual_available), product_obj.uom_id.name,
                     max(0,product_obj.qty_available), product_obj.uom_id.name)
            }
        # get unit price
        if not pricelist:
            warning = {
                'title': 'No Pricelist !',
                'message':
                    'You have to select a pricelist or a customer in the sales form !\n'
                    'Please set one before choosing a product.'
                }
        else:
            price = self.pool.get('product.pricelist').price_get(cr, uid, [pricelist],
                    product, qty or 1.0, partner_id, {
                        'uom': uom,
                        'date': date_order,
                        })[pricelist]
            if price is False:
                warning = {
                    'title': 'No valid pricelist line found !',
                    'message':
                        "Couldn't find a pricelist line matching this product and quantity.\n"
                        "You have to change either the product, the quantity or the pricelist."
                    }
            else:
                result.update({'price_unit': price})
        return {'value': result, 'domain': domain, 'warning': warning}

Yes i know i've already find this method, the problem is that i want to add into this method the two parameters i've added into the product.product model to return them into the result of the product_id_change method and permit to fill automatically two colums i've added to the sale.order.line.