Question

Django stated in their docs that all query sets are automatically cached, https://docs.djangoproject.com/en/dev/topics/db/queries/#caching-and-querysets. But they weren’t super specific with the details of this functionality.

The example that they gave was to save the qs in a python variable, and subsequent calls after the first will be taken from the cache.

queryset = Entry.objects.all()
print([p.headline for p in queryset]) # Evaluate the query set.
print([p.pub_date for p in queryset]) # Re-use the cache from the evaluation.

So even if two exact queryset calls were made without a variable subsequently when a user loads a view, would the results not be cached?

# When the user loads the homepage, call number one (not cached)
def home(request):    
    entries = Entry.objects.filter(something)
    return render_to_response(...)

# Call number two, is this cached automatically? Or do I need to import cache and
# manually do it? This is the same method as above, called twice
def home(request):    
    entries = Entry.objects.filter(something)
    return render_to_response(...)

Sorry if this is confusing, I pasted the method twice to make it look like the user is calling it twice, its just one method. Are entries automatically cached?

Thanks

Answer 1

The queryset example you have given rightly indicates that querysets are evaluated lazily i.e the first time they are used. So when subsequently used again, they are not evaluated in the same flow when assigned to a variable. This is not exactly caching but re-using an evaluated expression as long as it is available in an optimized manner.

For the kind of caching you are looking at i.e the same view called twice, you will need to manually cache the database object when it is fetched the first time. Memcached is good for this. Then subsequently check and fetch like in example below.

def view(request):
    results = cache.get(request.user.id)
    if not results:
        results = do_a_ton_of_work()
        cache.set(request.user.id, results)

There are of course a lot of other ways to do caching at different levels right from your proxy server to per url caching. Whatever works best for you. Here is a good read on this topic.