Create new invoice by code in another model Odoo 8

Question

I need to call the method write of account invoice from a button. I have a form with information and i want to create a draft invoice when the button is clicked passing the values for the invoice for parameter. Thanks

This my value :-

partner_id (cars_reservation)>>> partner_id (account.invoice)  

product_id (cars_reservation)>>> product_id (account.invoice.line)

product_price (cars_reservation)>>> price_unit (account.invoice.line)

price_total (cars_reservation)>>> amount_total (account.invoice)

---

this my Code class :-

class cars_reservation(models.Model): 

   _name = 'cars.reservation'
   _rec_name = 'name_reservation'

def action_draft(self, cr, uid, ids, context=None):
    return self.write(cr,uid,ids,{'state' : 'draft'})

def action_done(self, cr, uid, ids, context=None):
    return self.write(cr,uid,ids,{'state' : 'finished'})

def action_cancel(self, cr, uid, ids, context=None):
    return self.write(cr,uid,ids,{'state' : 'canceled'})


name_reservation = fields.Char('number', size=64, readonly=True)
reservation_date_time = fields.Datetime('Date')
partner_id = fields.Many2one('res.partner', string='Partner', required=True)
product_id = fields.Many2one('product.product', string='Product')
product_price = fields.Integer('price')
price_total = fields.Integer('price')
state = fields.Selection([
    ('new', 'new'),
    ('finished', 'finished'),
    ('canceled', 'canceled')
    ], 'state', readonly=True,        
    select=True)

_defaults = {

    'state': lambda * a:'new',
    'name_reservation': lambda self,cr,uid,context={}: self.pool.get('ir.sequence').get(cr, uid, 'reg_code'),

}