What is the best way to validate a json object? (POST request)

I must create a class (JobClass) and include ActiveModel::Validations?
and then
valided = JobClass.new(params).valid?

The JSON body :
http://pastebin.com/TwRbydmq

I can not create rules corresponding to the validation of the field
"method" ... It can be "concat" or "split" but it is not the first
level.

How can I do?

Thanks