ARGOMENTO: aggiungere stato al sale.order

from openerp.osv import fields, osv, orm

class sale_order(orm.Model):
    _inherit = 'sale.order'

    def __init__(self, cr, uid):
        super(sale_order, self).__init__(cr, uid)
        state = fields.selection(selection_add=[('prova', 'In Prova')])

from openerp import fields, models

class SaleOrder(models.Model):
    _inherit = 'sale.order'
    state = fields.selection(selection_add=[('prova', 'In Prova')])

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="view_order_caform" model="ir.ui.view">
            <field name="name">sale.order.form.view.ca</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
            	<data>
	                <field name="state" position="attributes">
                        <attribute name="statusbar_visible" >{"prova"}</attribute>
                    </field>
                </data>
            </field>
            
        </record>
    </data>
</openerp>