何时删除Django中的缓存条目

Question

In my django application ,I have a BlogEntry which belongs to a Category .A BlogEntry may belong to many Category s

class BlogEntry(models.Model):
    creationdate=models.DateField(default=date.today)    
    description=models.TextField()
    author=models.ForeignKey(User,null=True)
    categories=models.ManyToManyField(Category)

class Category(models.Model):
    name=models.CharField(unique=True,max_length=50)
    description=models.TextField(blank=True)

A user may edit a BlogEntry and in doing so, may remove a Category it was in.

Suppose blogEntry1 belonged to java,scala before .If user edits it such that he removes scala .Now the entry has only one category ie java

In my list_view I am using cache as below

from django.core.cache import cache
def list_entries_on_day(request,year,month,day):
    ...
    key = 'entries_day'+'-'+year+'-'+month+'-'+day
    if key not in cache:
        entries = BlogEntry.objects.filter(...args..)
        cache.set(key,entries)
    entries_on_day =cache.get(key)
    ...

Suppose I have created 2 entries for today and these are put in cache.If I edit one of these BlogEnty s and remove a category ie;

blogEntry1  has categories :java,scala
blogEntry2 has categories :dotnet,vbasic

Initially I make a query for entries for today and put the result in cache

cache now has [blogEntry1,blogEntry2] against key 'entries_day-2012-11-11'

Now I edit blogEntry1 such that it now has java as category

,do I need to remove the stored entries from cache?(Since the cache contains a BlogEntry object before it's modification)

Answer 1

You can invalidate cache by registering a signal handler for model.save

You can also live with the fact that users will see the stale content until the cache expiration (1 hour default) make sure he logged in user will not see the cached content, otherwise he will honk the edit was lost.

Hmmm my answer is a bit vague, but I just wanted to say: no, you don't strictly have to invalidate cache at each edit, it is a choice between performance and content freshness.

One more nit: the preferred idiom for cache usage is:

entries_on_day = cache.get(key)
if entries_on_day  is None:
    entries_on_day  = BlogEntry.objects.filter(...args..)
    cache.set(key,entries_on_day)

You save one cache query