I want to notify whenever one 'comment' is added in backend to any sale order. To do some actions in the website (I have created one view to show sale orders information).

This is mi code:
@api.model    
def create(self, values):        
    if values.get('model') == 'sale.order' and values.get('message_type') == 'comment':            
        channel = (self._cr.dbname, 'sale.order', 3)            
        messages = {'type': 'user_notification', 'title': 'Notifiy User'}            
        self.env['bus.bus'].sendone(channel, messages)        
    return super(MailMessage, self).create(values)                    

And I added the channel before like this:

class UserPortalBusController(BusController):    
def _poll(self, dbname, channels, last, options):        
    """Add the relevant channels to the BusController polling."""        
    if request.session.uid:            
        channels = list(channels)            
        channels.append((request.db, 'sale.order', 3))                
    return super(UserPortalBusController, self)._poll(dbname, channels, last, options)            

When I add one message to sale order I am having this error:

File "/usr/lib/python3/dist-packages/odoo/api.py", line 394, in _model_create_single
    return self.browse().concat(*(create(self, vals) for vals in arg))
  File "/usr/lib/python3/dist-packages/odoo/api.py", line 394, in 
    return self.browse().concat(*(create(self, vals) for vals in arg))
  File "/mnt/addons_bp/user_portal/models/mail_message.py", line 18, in create
    self.env['bus.bus'].sendone(channel, messages)
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 644, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 302, in _handle_exception
    raise exception.with_traceback(None) from new_cause
AttributeError: 'bus.bus' object has no attribute 'sendone'