gistfile1.rb

class ApplicationController < ActionController::Base
  before_filter :merge_json_params

private

  def merge_json_params
    if request.format.json?
      body = request.body.read
      request.body.rewind
      params.merge!(ActiveSupport::JSON.decode(body)) unless body == ""
    end
  end
  
end

This is a really nice gist for a really icky problem without a good solution on rails 3, so thumbs up! But in order to solve the issue for parameters wrapped in the model name I suggest modifying it to:

def fix_json_params
  if request.format.json?
    body = request.body.read
    request.body.rewind
    unless body == ""
      unmunged_body = ActiveSupport::JSON.decode(body)
      params.merge!(unmunged_body)
      params[_wrapper_options[:name]].merge!(unmunged_body)
    end
  end
end