db_validating_cache_machine.py

class DbValidatingCacheMixin(object):
    """
    Cache mixin that avoids returning database objects from the cache that
    reference invalid databases (databases that no longer exist in the settings
    file).
    """

    def _validate(self, key, obj, default=None):
        """
        If ``obj`` appears to be a model object or list of model objects that
        references a database that no longer exists in the settings file, return
        ``default`` and delete ``key`` from the cache.
        """
        # if the object is an iterable, check the first element in that
        # iterable. otherwise, check the object itself
        try:
            check_obj = next(iter(obj))
        except TypeError:
            check_obj = obj
        except StopIteration:
            return obj
        if hasattr(check_obj, '_state') and hasattr(check_obj._state, 'db'):
            if check_obj._state.db is None or check_obj._state.db in settings.DATABASES.keys():
                return obj
            else:
                logger.warning('Deleting cache key "%s" because it references an '
                               'invalid db (%s)' % (key, check_obj._state.db))
                self.delete(key)
                return default
        else:
            return obj

    def get(self, key, default=None, version=None):
        """
        Retrieve a value from the cache.
        """
        result = super(DbValidatingCacheMixin, self).get(key, default, version)
        return self._validate(key, result, default)

    def get_many(self, keys, version=None):
        """
        Retrieve many keys.
        """
        result = super(DbValidatingCacheMixin, self).get_many(keys, version)
        bad_keys = []
        for k, v in result.iteritems():
            if v is not None and self._validate(k, v) is None:
                bad_keys.append(k)
        for k in bad_keys:
            result.pop(k)
        return result