code: def exists_locally?(key_name)
        return true if @cache.key?(key_name) || @writethrough.key?(key_name)
        meta = ensure_metadata_exists(key_name)
        @source.key?(meta.source_key)
      end
docs: Whether the key doesn't need to be lazily loaded.
   @param key_name [Symbol] the key to check
   @return [Boolean] whether the key exists locally.