[Feature request] Support custom FromJSON and ToJSON for entity keys?

Hello.

I use Yesod + Persistent to build my application, and I'm planning to use hashids for primary key representations, since I've thought it would be nice if the traditional integer ids would be hidden from users.

I also tried to make entity keys (such as Key User) an instance of FromJSON and ToJSON, which encodes to / decodes from hashid representations (i.e. String constructor of aeson's Value type), but I realized that Database.Persist.TH made them instances of FromJSON and ToJSON type, whose values are treated as Number.

I read the haddock document, but I couldn't find the way to provide custom instance declaration for them.
(I've found mpsEntityJSON, which allow us to work with Entity a, but what I've wanted is the one for Key a.)

Thanks.

Start by reading the Database.Persist.TH Haddock and the generated FromJSON and ToJSON instances for entity keys. Compare that behavior with mpsEntityJSON and determine how custom JSON handling for Key a could be exposed. Done means users can provide hashid-style String encoding and decoding for entity keys without the default Number representation.