You could just pass in the params hash here and render json for the delta.
```ruby
answer_params = params.dup
answer_params[:delta] = answer_params[:delta].to_json
```

But, also, is it the controller responsibility to know that Answer stores the delta as json?  Perhaps it should be Answer models responsibility to convert delta.  

If delta is just a hash,  you can use [serialize](http://api.rubyonrails.org/classes/ActiveModel/Serialization.html) and let ActiveRecord handle the conversion for you!