Question

I am using Django. I am having a few issues with caching of QuerySets for news/category models:

class Category(models.Model):
    title = models.CharField(max_length=60)
    slug = models.SlugField(unique=True)

class PublishedArticlesManager(models.Manager):
    def get_query_set(self):
        return super(PublishedArticlesManager, self).get_query_set() \
                    .filter(published__lte=datetime.datetime.now())

class Article(models.Model):
    category = models.ForeignKey(Category)

    title = models.CharField(max_length=60)
    slug = models.SlugField(unique = True)
    story = models.TextField()
    author = models.CharField(max_length=60, blank=True)
    published = models.DateTimeField(
        help_text=_('Set to a date in the future to publish later.'))
    created = models.DateTimeField(auto_now_add=True, editable=False)
    updated = models.DateTimeField(auto_now=True, editable=False)

    live = PublishedArticlesManager()
    objects = models.Manager()

Note – I have removed some fields to save on complexity…

There are a few (related) issues with the above.

Firstly, when I query for LIVE objects in my view via Article.live.all() if I refresh the page repeatedly I can see (in MYSQL logs) the same database query being made with exactly the same date in the where clause – ie – the datetime.datetime.now() is being evaluated at compile time rather than runtime. I need the date to be evaluated at runtime.

Secondly, when I use the articles_set method on the Category object this appears to work correctly – the datetime used in the query changes each time the query is run – again I can see this in the logs. However, I am not quite sure why this works, since I don’t have anything in my code to say that the articles_set query should return LIVE entries only!?

Finally, why is none of this being cached?

Any ideas how to make the correct time be used consistently? Can someone please explain why the latter setup appears to work?

Thanks
Jay

P.S – database queries below, note the date variations.

SELECT LIVE ARTICLES, query #1:

SELECT `news_article`.`id`, `news_article`.`category_id`, `news_article`.`title`, `news_article`.`slug`, `news_article`.`teaser`, `news_article`.`summary`, `news_article`.`story`, `news_article`.`author`, `news_article`.`published`, `news_article`.`created`, `news_article`.`updated` FROM `news_article` WHERE `news_article`.`published` <= '2011-05-17 21:55:41'  ORDER BY `news_article`.`published` DESC, `news_article`.`slug` ASC;

SELECT LIVE ARTICLES, query #1:

SELECT `news_article`.`id`, `news_article`.`category_id`, `news_article`.`title`, `news_article`.`slug`, `news_article`.`teaser`, `news_article`.`summary`, `news_article`.`story`, `news_article`.`author`, `news_article`.`published`, `news_article`.`created`, `news_article`.`updated` FROM `news_article` WHERE `news_article`.`published` <= '2011-05-17 21:55:41'  ORDER BY `news_article`.`published` DESC, `news_article`.`slug` ASC;

CATEGORY SELECT ARTICLES, query #1:

SELECT `news_article`.`id`, `news_article`.`category_id`, `news_article`.`title`, `news_article`.`slug`, `news_article`.`teaser`, `news_article`.`summary`, `news_article`.`story`, `news_article`.`author`, `news_article`.`published`, `news_article`.`created`, `news_article`.`updated` FROM `news_article` WHERE (`news_article`.`published` <= '2011-05-18 21:21:33'  AND `news_article`.`category_id` = 1 ) ORDER BY `news_article`.`published` DESC, `news_article`.`slug` ASC;

CATEGORY SELECT ARTICLES, query #1:

SELECT `news_article`.`id`, `news_article`.`category_id`, `news_article`.`title`, `news_article`.`slug`, `news_article`.`teaser`, `news_article`.`summary`, `news_article`.`story`, `news_article`.`author`, `news_article`.`published`, `news_article`.`created`, `news_article`.`updated` FROM `news_article` WHERE (`news_article`.`published` <= '2011-05-18 21:26:06'  AND `news_article`.`category_id` = 1 ) ORDER BY `news_article`.`published` DESC, `news_article`.`slug` ASC;

Answer 1

I have now fixed this issue. It appears the problem was that the queryset returned by Article.live.all() was being cached in my urls.py! I was using function-based generic-views:

url(r'^all/$', object_list, {
        'queryset' : Article.live.all(),
    }, 'news_all'),

I have now changed this to use the class-based approach, as advised in the latest Django documentation:

url(r'^all/$', ListView.as_view(
        model=Article,
    ), name="news_all"),

This now works as expected – by specifying the model attribute rather than the queryset attribute the query is QuerySet is created at compile-time instead of runtime.