django: how to memoize model manager methods?

Question

I have a memoized Django model manager method as follows:

class GroupManager(models.Manager):
    def get_for_user(self, user):
        cache_key = 'groups_%s' % (user.id)
        if not hasattr(self, key):
            groups = get_groups_somehow()
            setattr(self, cache_key, groups)
        return getattr(self, cache_key)

But the memoized value persists beyond the request / response cycle; i.e. the value is not re-calculated in the subsequent requests until the server is restarted. This must be because the manager instance is not destroyed.

So, how can I properly memoize model manager methods?

Answer 1

with inspiration from https://stackoverflow.com/a/1526245/287923, but simplifying it, i've implemented a request cache as follows:

from threading import currentThread

caches = {}

class RequestCache(object):
    def set(self, key, value):
        cache_id = hash(currentThread())
        if caches.get(cache_id):
            caches[cache_id][key] = value
        else:
            caches[cache_id] = {key: value}

    def get(self, key):
        cache_id = hash(currentThread())
        cache = caches.get(cache_id)
        if cache:
            return cache.get(key)
        return None

class RequestCacheMiddleware(object):
    def process_response(self, request, response):
        cache_id = hash(currentThread())
        if caches.get(cache_id):
            del(caches[cache_id])
        return response

caches is a dictionary of cache dictionaries, accessed via get & set methods. a middleware clears the cache for the current thread in process_response method, after the response is rendered.

it is used like this:

from request_cache import RequestCache
cache = RequestCache()
cache.get(key)
cache.set(key, value)