<p>I have a pretty generic Article model, with m2m relation to Tag model. I want to keep count of each tag usage, i think the best way would be to denormalise count field on Tag model and update it each time Article being saved. How can i accomplish this, or maybe there's a better way?</p>

You can do this by creating an
intermediate model
for the M2M relationship and use it as your hook for the
```
post_save
```
and
```
post_delete
```
signals to update the denormalised column in the
```
Article
```
table.
For example, I do this for favourited
```
Question
```
counts in
soclone
, where
```
User
```
s have a M2M relationship with
```
Question
```
s:
```
```
from django.contrib.auth.models import User
from django.db import connection, models, transaction
from django.db.models.signals import post_delete, post_save

class Question(models.Model):
    # ...
    favourite_count = models.PositiveIntegerField(default=0)

class FavouriteQuestion(models.Model):
    question = models.ForeignKey(Question)
    user     = models.ForeignKey(User)

def update_question_favourite_count(instance, **kwargs):
    """
    Updates the favourite count for the Question related to the given
    FavouriteQuestion.
    """
    if kwargs.get('raw', False):
        return
    cursor = connection.cursor()
    cursor.execute(
        'UPDATE soclone_question SET favourite_count = ('
            'SELECT COUNT(*) from soclone_favouritequestion '
            'WHERE soclone_favouritequestion.question_id = soclone_question.id'
        ') '
        'WHERE id = %s', [instance.question_id])
    transaction.commit_unless_managed()

post_save.connect(update_question_favourite_count, sender=FavouriteQuestion)
post_delete.connect(update_question_favourite_count, sender=FavouriteQuestion)

# Very, very naughty
User.add_to_class('favourite_questions',
                  models.ManyToManyField(Question, through=FavouriteQuestion,
                                         related_name='favourited_by'))
```
```
There's been a bit of discussion on the django-developers mailing list about implementing a means of declaratively declaring denormalisations to avoid having to write code like the above:
Denormalisation, magic, and is it really that useful?
Denormalisation Magic, Round Two