django-counter-field

It is sometimes useful to cache the total number of objects associated with another object through a ForeignKey relation. For example the total number of comments associated with an article.

django-counter-field makes it easy to denormalize and keep such counters up to date.

Quick start

1. Install django-counter-field:

   pip install django-counter-field
   

2. Add "django_counter_field" to your INSTALLED_APPS setting:

   INSTALLED_APPS = (
       ...
       'django_counter_field',
   )
   

3. Add a CounterField to your model:

   from django_counter_field import CounterField
   
   
   class Article(models.Model):
       comment_count = CounterField()
   

4. Add the CounterMixin to the related model:

   from django_counter_field import CounterMixin, connect_counter
   
   
   class Comment(CounterMixin, models.Model):
       article = models.ForeignKey(Article)
   

5. Connect the related foreign key field with the counter field:

   connect_counter('comment_count', Comment.article)
   

Whenever a comment is created the comment_count on the associated Article will be incremented. If the comment is deleted, the comment_count will be automatically decremented.

Overview

Creating a new counter requires three simple steps:

1. Add a CounterField field to the parent model.
2. Add the CounterMixin mixin to the child model.
3. Use connect_counter to connect the child model with the new counter.

Most counters are simple in the sense that you want to count all child objects. Sometimes, however, objects should be counted based on one or several conditions. For example you may not wish to count all comments on an article but only comments that have been approved. You can create conditional counters by providing a third is_in_counter_func argument to connect_counter:

connect_counter('comment_count', Comment.article, lambda comment: comment.is_approved)

The is_in_counter_func function will be called with Comment objects and must return True if the given comment should be counted. It must not concern itself with checking if the comment is deleted or not, django-counter-field does that by default.

Backfilling

Often you will add a CounterField to a model that already has a large number of associated objects. When a counter is created, it's value is initialized to zero. This value is likely incorrect. django-counter-field provides a couple of management commands that allow you to rebuild the value of a counter:

1. List all available counters:

   $ python manage.py list_counters

2. Rebuild a counter using one of the counter names given by list_counters:

   $ python manage.py rebuild_counter <counter_name>

Note: The rebuild_counter management command will only update counters on objects that have at least one child object. For example articles with at least one comment. Articles with no comments will not be updated. This is a conscious limitation; the use cases for such a feature seem very limited, if existent at all.

Documentation

$ pip install Sphinx $ cd docs $ make html Open build/html/index.html