relates to #397 since IMHO complicates things upon API changes.
I wonder if we should work out an additional option for get(), like strict defaulting to True (or False for backward compatibility although not sure if desired).
ATM get just returns an empty list if I provide absent/miss-specified entity/metadata. API is also in flux, e.g. in 0.8 we now got scope instead of domain for get(). So whatever was working before for domain="derivatives" doesn't work now as expected -- get() just returns an empty list since no match in entities/metadata for such a domain and we cannot really even alert a user that may be he is relying on previous API since we don't know if that was not intended to be entity/metadata (that is what I meant above as a relation to #397). As a user I would just spend considerable amount of time to figure out WTF got changed or how my dataset is "wrong" if I just started to query a new one.

I would be happy to work out an initial implementation. For checking entities it is easy, but not sure how to verify that metadata fields are anyhow known at all