Extracted from Django docs: “memory isn’t intended for permanent data storage, so don’t rely on memory-based caching as your only data storage. Without a doubt, none of the Django caching backends should be used for permanent storage”.
Here is the lesson: don’t rely on caching. Cache doesn’t exist to make applications work, it exists to make them faster and/or more scalable. Use a cache system as permanent storage is such a bad idea, except when you’re not using it for caching (what is not the case here). Cache is a shortcut for a path that might be painful to follow, but you still should not rely on cache. You must be able to traverse the path without the cache.