Every blogger is interested in comments for his/her blog posts. So do I. Unfortunately there are no so much comments on my blog (only two), but I hope there will be more in future :)

But how could the author of the blog be notified about new comments? One possibility is to create feed for new comments and subscribe to it using your favourite feed reader (f.e. Google Reader). In fact I had been using this approach until recently. But I think that more convenient approach is to be notified by email. Now I am going to describe how I implemented this functionality in my blog.

The problem seems to be simple – just send email after comment object is saved to database. But I am using django.contrib.comments application and can not modify its code.

Fortunately django provides way to hook into process of the object saving. Internally Django (or more strictly speaking post magic-removal version of Django) uses PyDispatch framework – multiple-producer-multiple-consumer signal-dispatching system :) Despite this complex name idea of this framework is really simple: you can produce signals using following method

dispatcher.send(signal=Any, sender=Anonymous,
    *args, **kwargs)

and register listeners for the signals using:

dispatcher.connect(receiver, signal=Any, sender=Any,
    weak=True)

Check sources of django.dispatch package for more details.

Django ORM framework supports following signals defined in django.db.models.signals package:

signal	when given signal is sent	args	kwargs
pre_init	before model object initialization (from constructor)	positional arguments to init	keyword arguments to init
post_init	after object is initialized	None	instance = self
pre_save	just before saving object to database	None	instance = self
post_save	after saving object to database	None	instance = self
pre_delete	just before deleting object from database	None	instance = self
post_delete	after deleting object from database	None	instance = self

For all these signals model class serves as sender.

Now our task becomes very easy – write and register post_save signal listener for FreeComment class:

from django.db import models
from django.db.models import signals
from django.dispatch import dispatcher
from django.core.mail import send_mail
from django.template import Context, loader, Template,
    TemplateDoesNotExist
from django.contrib.comments.models import FreeComment

# send mail on comment
def send_comment_by_mail(instance):
  comment = instance
  if comment.content_type.model_class() != Entry:
    return
  entry = Entry.objects.get(id__exact = comment.object_id)

  # templates for mail subject and body
  try:
    subject_tmp = loader.get_template("free_comment_subject")
  except TemplateDoesNotExist:
    subject_tmp = Template('New comment for entry \
       "{{ entry.title }}" by "{{ comment.person_name }}"')
  try:
    body_tmp = loader.get_template("free_comment_body")
  except TemplateDoesNotExist:
    body_tmp = Template('{{ comment.comment }}')

  # send email to the user
  ctx = Context({'entry': entry, 'comment': comment})
  subject = subject_tmp.render(ctx).decode('utf-8').strip()
  body = body_tmp.render(ctx).decode('utf-8')
  entry.author.email_user(subject, body)

# connect signal
dispatcher.connect(send_comment_by_mail,
    sender = FreeComment,
    signal = signals.post_save)

Posted by on July 1, 2006  |  development, django

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