@api.multi
def invoice_validate(self):
  ...

@api.multi
def invoice_validate(self):
    for invoice in self:
        ...
        origin = self.origin
        sale_order_id = self.env['sale.order'].search([('name', '=', origin)])[0].id
        sale_order_obj = self.env['sale.order'].browse(sale_order_id)
        sale_order_obj.write({'invoice_status': 'invoiced'})
    return self.write({'state': 'open'})

invoice_status = fields.Selection([
    ('upselling', 'Upselling Opportunity'),
    ('invoiced', 'Fully Invoiced'),
    ('to invoice', 'To Invoice'),
    ('no', 'Nothing to Invoice')
    ], string='Invoice Status', compute='_get_invoiced', store=True, readonly=True, default='no')

invoice_status = fields.Selection(selection_add=[("state", "open")])