write an onchange method in the model and use that one from the xml form view to alter your fields.

here's an example from sale.py:

def onchange_pricelist_id(self, cr, uid, ids, pricelist_id, order_lines, context=None):
        context = context or {}
        if not pricelist_id:
            return {}
        value = {
            'currency_id': self.pool.get('product.pricelist').browse(cr, uid, pricelist_id, context=context).currency_id.id
        }
        if not order_lines:
            return {'value': value}
        warning = {
            'title': _('Pricelist Warning!'),
            'message' : _('If you change the pricelist of this order (and eventually the currency), prices of existing order lines will not be updated.')
        }
        return {'warning': warning, 'value': value}

In the sale Order selecing the product it will automatically calling, try this,

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, context=None): context = context or {} lang = lang or context.get('lang',False) if not partner_id: raise osv.except_osv(_('No Customer Defined !'), _('Before choosing a product,\n select a customer in the sales form.')) warning = {} product_uom_obj = self.pool.get('product.uom') partner_obj = self.pool.get('res.partner') product_obj = self.pool.get('product.product') context = {'lang': lang, 'partner_id': partner_id} if partner_id: lang = partner_obj.browse(cr, uid, partner_id).lang context_partner = {'lang': lang, 'partner_id': partner_id}

    if not product:
        return {'value': {'th_weight': 0,
            'product_uos_qty': qty}, 'domain': {'product_uom': [],
               'product_uos': []}}
    if not date_order:
        date_order = time.strftime(DEFAULT_SERVER_DATE_FORMAT)

    result = {}
    warning_msgs = ''
    product_obj = product_obj.browse(cr, uid, product, context=context_partner)

    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
    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['tax_id'] = self.pool.get('account.fiscal.position').map_tax(cr, uid, fpos, product_obj.taxes_id)

    if not flag:
        result['name'] = self.pool.get('product.product').name_get(cr, uid, [product_obj.id], context=context_partner)[0][1]
        if product_obj.description_sale:
            result['name'] += '\n'+product_obj.description_sale
    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
    # get unit price

    if not pricelist:
        warn_msg = _('You have to select a pricelist or a customer in the sales form !\n'
                'Please set one before choosing a product.')
        warning_msgs += _("No Pricelist ! : ") + warn_msg +"\n\n"
    else:
        price = self.pool.get('product.pricelist').price_get(cr, uid, [pricelist],
                product, qty or 1.0, partner_id, {
                    'uom': uom or result.get('product_uom'),
                    'date': date_order,
                    })[pricelist]
        if price is False:
            warn_msg = _("Cannot find a pricelist line matching this product and quantity.\n"
                    "You have to change either the product, the quantity or the pricelist.")

            warning_msgs += _("No valid pricelist line found ! :") + warn_msg +"\n\n"
        else:
            result.update({'price_unit': price})
    if warning_msgs:
        warning = {
                   'title': _('Configuration Error!'),
                   'message' : warning_msgs
                }
    return {'value': result, 'domain': domain, 'warning': warning}

sale_view.xml

on_change="product_id_change(parent.pricelist_id, product_id, product_uom_qty, product_uom, product_uos_qty, product_uos, name, parent.partner_id, False, True, parent.date_order, False, parent.fiscal_position, False, context)"/>

try this procedure it will be usefull.