gistfile1.rb

  class ExampleController < ApplicationController

    def create
      # something that calls private method
      # ...
    end

    private
    def private_method
      params.
        fetch(:top_level).
        fetch(0).
        fetch(:another_attribute)
    rescue
       # do something that let's us
       # render text: "Invalid query parameters", status: :unprocessable_entity
       # at the top level
    end
  end

Rails will issue a 422 : Unprocessable Entity from the respond_with handler if the object you give it is invalid. Does that help?

Not in this specific case.

I might have the private method return either the real object or a benign object which can then be used as normal. i.e., the private method is just a factory to map from the web (HTTP params) to your domain. Meh?