ActiveModel Serializers with custom root bits

ams_custom_roots.rb

require File.expand_path("../config/environment", __FILE__)
require 'rspec'

# Disable for all serializers (except ArraySerializer)
ActiveModel::Serializer.root = false
#
# # Disable for ArraySerializer
ActiveModel::ArraySerializer.root = false

class AMSEnvelope
  attr_accessor :metadata, :errors, :result

  def initialize(meta, err, res)
    self.metadata = meta
    self.errors = err
    self.result = serializer(res).new(res).as_json
  end

  def read_attribute_for_serialization(attr)
    send(attr)
  end

  def serializer(obj)
    "#{obj.class.name}Serializer".safe_constantize ||
      ActiveModel::DefaultSerializer
  end
end

class AMSEnvelopeSerializer < ActiveModel::Serializer
  attributes :metadata, :errors, :result
end

class GroupSerializer < ActiveModel::Serializer
  attributes :name
end

describe "AMS with envelope" do
  let(:envelope) { AMSEnvelope.new({ current_user: {} }, [], Group.new(name: "test")) }
  subject { AMSEnvelopeSerializer.new(envelope) }

  it "renders all the things" do
    subject.as_json.should == { metadata: { current_user: {} }, errors: [], result: { name: "test" } }
  end
end