I made a new module from sales.order (odoo11).  I have a field "cantidad" that should show the number of lines in the order but is not working .. not errors but not work

I try sereral example but the computed field desn't work

 here is the code 

from odoo import api, fields, models

class aleOrder(models.Model):

    _name = "ale.order"

    _description = "Forma"

    name = fields.Char(string='OT')

    partner_id = fields.Many2one('res.partner', string='Cliente',  required=True,  track_visibility='always')

       order_line = fields.One2many('ale.order.line', 'order_id', string='Trabajos', copy=True, auto_join=True)

    cantidad = fields.Float(string='Extintores', store='True', readonly='True', compute='_cant_ext')

@api.depends('order_line')

def _cant_ext(self):

        for line in self:

            cantidad += line.sequence

        order.update({

        'cantidad': cantidad,

        })

class aleOrderLine(models.Model):

    _name = 'ale.order.line'

    _description = 'Lines'

    order_id = fields.Many2one('ale.order', string='Order Reference', required=True, ondelete='cascade', index=True, copy=False)

    name = fields.Text(string='Description')

    sequence = fields.Float(string='Sequence', default=10)

any help ?