How can I get a key for a fetch_by_some_attribute([a, b]) if the result is nil?

[SubandiK]

So I'm running into stale data issues as per the warning in the readme.
I would like to just invalidate / not even store the result if a fetch_by_some_attribute returns nil.
Failing that, I would like to invalidate the cache by doing IdentityCache.cache.clear(some_key).
But how do I get the key from
Model.fetch_by_some_attribute(['green', 'red'])
if it returns nil?