Render model to json-schema?

http://stackoverflow.com/questions/3068792/how-can-i-export-a-rails-model-to-json-schema

I am looking to optimize how we build forms for some of our models and ideally I would like to build them from json-schema.

Is there a gem or the like which would allow me to export a model definition to json-schema?

Bonus: With validations.

Bonus: While modelling association relationships.