gistfile1.rb

class ActiveRecord::Base
  cattr_accessor :my_json_attributes_key
  self.set_my_json_attributes_key = :apiv1
end

class Api::V2::Controller < ApplicationController
  around_filter :set_my_json_attributes_key
  ...
  private
  def set_my_json_attributes_key
    old = ActiveRecord::Base.my_json_attributes_key
    ActiveRecord::Base.my_json_attributes_key = :apiv2
    yield
  ensure
    ActiveRecord::Base.my_json_attributes_key = old
  end
end

class Bookmark < ActiveRecord::Base
  JSON_ATTRS = {
    :apiv1 => [:id, :owner_id, :owner_type, :url, :name, :position],
    :apiv2 => [:id, :owner_id, :owner_type, :url, :name, :position, :foo, :bar]
  }
  def as_json(options=nil)
    attributes.slice(*JSON_ATTRS[my_json_attributes_key])
  end
end