related_caching.rst

Related Object Caching

Introduction

Given a model instance, one frequent database query is to get instances of another model that are related. This is commonly accomplished with the use of a Foreign Key.

We will be using the following models as examples throughout this document: :

from django.db import models
from cachemagic.controllers import RelatedCacheController

class Person(models.Model):
    name = models.CharField(max_length=200)

    cache = RelatedCacheController()

class Book(models.Model):
    author = models.ForeignKey(Person, related_name='books')
    title = models.CharField(max_length=200)
    published_date = models.DateTimeField()

    class Meta:
        default_ordering = ('-published_date')

Suppose you have an authorship view, displaying all of the books that a given author has published. The view would typically look something like this: :

def authorship(request, author_id):
    try:
        author = Person.objects.get(pk=author_id)
    except Person.DoesNotExist:
        raise Http404("No such person")
    books = author.books.all()
    return render(
        request,
        {'author': author, 'books': books},
        'authorship.html'
    )

This pattern will invoke two database queries: one to fetch a Person, and one to fetch the books with a foreign key relationship to the author. We can use the cachemagic features to try the cache first. :

author = Person.objects.get(pk=author_id)   # database query
author = Person.cache.get(pk=author_id)     # cached query

books = author.books.all()                  # database query
books = author.cache.books                  # cached query

Reading From Cache

Given an instance of an object with a RelatedCacheController, all of the attributes on the instance to fetch related objects are mirrored on the controller. If the instance has a .thing_set and a RelatedCacheManager assigned to cache, then instance.cache.thing_set will return the same values as list(instance.thing_set.all()).

Note

Related object caches return lists of instances, not querysets. This means that you don't need to put the .all() on the end, but also that you can not apply django queryset operations like .filter() or .select_related() on the result.

Cache Keys

A cache key for the instance is obtained by calling the same make_key(pk) function described in instance_cache_keys. The key for the related objects is the instance key, appended with the related name of the collection. :

author = Person.objects.get(pk=1)   # get an instance of a Person in the sample_app
author.cache.books                  # cache key is sample_app:Person:1:books

Cache Timeouts and Multicache

The RelatedCacheController accepts the same timeout <cache_timeouts> and backend <multicache> arguments as CacheController.

cache = RelatedCacheController(
        timeout=(60 * 60 * 24 * 7),     # timeout in one week
        backend='my_app_cache'),        # use the cache backend named 
                                        # 'my_app_cache' in settings.py
    )