How to create a full Audit log in Rails for every table?

We recently began a compliance push at our company and are required to keep a full history of changes to our data which is currently managed in a Rails application. We've been given the OK to simply push something descriptive for every action to a log file, which is a fairly unobtrusive way to go.

My inclination is to do something like this in ApplicationController:

around_filter :set_logger_username

def set_logger_username
  Thread.current["username"] = current_user.login || "guest"
  yield
  Thread.current["username"] = nil
end

Then create an observer that looks something like this:

class AuditObserver < ActiveRecord::Observer
  observe ... #all models that need to be observed

  def after_create(auditable)
    AUDIT_LOG.info "[#{username}][ADD][#{auditable.class.name}][#{auditable.id}]:#{auditable.inspect}"
  end

  def before_update(auditable)
    AUDIT_LOG.info "[#{username}][MOD][#{auditable.class.name}][#{auditable.id}]:#{auditable.changed.inspect}"
  end

  def before_destroy(auditable)
    AUDIT_LOG.info "[#{username}][DEL][#{auditable.class.name}][#{auditable.id}]:#{auditable.inspect}"
  end

  def username
    (Thread.current['username'] || "UNKNOWN").ljust(30)
  end
end

and in general this works great, but it fails when using the "magic" <association>_ids method that is tacked to has_many :through => associations.

For instance:

# model
class MyModel
  has_many :runway_models, :dependent => :destroy
  has_many :runways, :through => :runway_models
end

#controller
class MyModelController < ApplicationController

  # ...

  # params => {:my_model => {:runways_ids => ['1', '2', '3', '5', '8']}}

  def update
    respond_to do |format|
      if @my_model.update_attributes(params[:my_model])
        flash[:notice] = 'My Model was successfully updated.'
        format.html { redirect_to(@my_model)