Correct way to exclude attributes from json serialization


I'm using Mongoid and Rails 3, and I want to serialize one of my
mongoid documents to json for publishing via a controller, e.g.

class ReportController < ApplicationController
    respond_to :json

    def show
        report = Report.where(:report_id => params[:id]).first
        respond_with report

This works ok, but pushes out every attribute on the document
including the "internal" mongodb document id, e.g. :

    _id: 54857324987546, // internal mongoid document id
    report_field_a: 1,
    report_field_b: 2,

I would like to strip this when serializing the report to json, and I
would also like any embedded documents to have their _id properties
excluded too. I have searched for info on as_json, to_json etc but I'm
not sure which is the best practice approach. The closest I can get

class Report
  include Mongoid::Document
  def as_json(options = {})
    serializable_hash({ :except => :_id }.merge(options))

...and then I need to copy that same as_json definition onto embedded
document definitions too. Is this the rails 3 way of doing
serialization customization?

I tried defining as_json as:

  def as_json(options = {})
    super({ :except => :_id }.merge(options))

but that doesn't return a hash as I was expecting, it seems to return
the document itself. I'm not entirely sure what's happening there.

Any pointers are greatfully received!