Initializes local cache from Django cache if required.

def _cache_init(self):
        """Initializes local cache from Django cache if required."""
        cache_ = cache.get(self.CACHE_ENTRY_NAME)

        if cache_ is None:
            categories = get_category_model().objects.order_by('sort_order')

            ids = {category.id: category for category in categories}
            aliases = {category.alias: category for category in categories if category.alias}

            parent_to_children = OrderedDict()  # Preserve aliases order.
            for category in categories:
                parent_category = ids.get(category.parent_id, False)
                parent_alias = None
                if parent_category:
                    parent_alias = parent_category.alias
                if parent_alias not in parent_to_children:
                    parent_to_children[parent_alias] = []
                parent_to_children[parent_alias].append(category.id)

            cache_ = {
                self.CACHE_NAME_IDS: ids,
                self.CACHE_NAME_PARENTS: parent_to_children,
                self.CACHE_NAME_ALIASES: aliases
            }

            cache.set(self.CACHE_ENTRY_NAME, cache_, self.CACHE_TIMEOUT)

        self._cache = cache_