account.rb

module Canvas
  class Account < Canvas::Model
    Canvas::Model.setup

    attr_reader :instances

    field :id
    field :first_name, :required => true
    field :last_name, :required => true
    field :email, :required => true
    field :address, :required => true
    field :city, :required => true
    field :state, :required => true
    field :postal, :required => true
    field :country, :required => true
    field :organization
    field :status
    field :aws_access_key
    field :aws_secret_access_key

    def reload(options={})
      self.class.find(self.id)
    end

    def users
      refresh_users if @users.nil?
      @users
    end

    private
    
    def refresh_users
      @users = []
      response = begin
                 RestClient.get self.uri(:action => 'users', :id => self.id),
                   :content_type => :json, :accept => :json
               rescue RestClient::Unauthorized
                 nil
               rescue RestClient::ResourceNotFound
                 nil
               end

      unless response.nil?
        @users = JSON.parse(response.body).collect {|x| User.new(x)}
      end
      
      @users
    end

    def process(attrs = nil)
      @instances ||= []
      unless attrs.nil?
        if attrs.is_a? Hash
          (attrs['instances'] || attrs[:instances] || []).each do |i|
            @instances.push Instance.new(i)
          end
          attrs.delete 'instances'
          attrs.delete :instances
        end
      end
      super(attrs)
      refresh_users
    end
  end
end

instance.rb

module Canvas
  class Instance < Canvas::Model
    Canvas::Model.setup

    field :id
    field :serverid
    field :serverimage, :required => true
    field :serverflavor, :required => true
    field :serveripaddress
    field :serverprivateip
    field :serverstate

  end
end

output.txt

$ ruby testit.rb 
account:
{
  "id": "4c89e2450ad0559327000001",
  "first_name": "Steven",
  "last_name": "Steversen",
  "email": "[email protected]",
  "address": "123 Any St.",
  "city": "Awesomeville",
  "state": "MI",
  "postal": "48911",
  "country": "US",
  "organization": "Applied Awesome",
  "status": "active",
  "aws_access_key": null,
  "aws_secret_access_key": null
}
account.instances:
[
  "4c986f100ad05530cf000001"
]
account.users:
[
  "4c89e2450ad0559327000003"
]

testit.rb

$:.unshift 'lib'

require 'canvas'

Canvas.register_api_key 'THISISCOMPLETELYRANDOM'

account = Canvas::Account.find('4c89e2450ad0559327000001')

puts "account:"
puts JSON.pretty_generate(account.attributes)

puts "account.instances:"
puts JSON.pretty_generate(account.instances.collect {|x| x.id })

puts "account.users:"
puts JSON.pretty_generate(account.users.collect {|x| x.id })

user.rb

module Canvas
  class User < Canvas::Model
    Canvas::Model.setup

    field :id
    field :login, :required => true
    field :password
    field :api_key

    private
    def process(attrs)
      @account = nil
      unless attrs.nil?
        if attrs.is_a? Hash
          (attrs['account_id'] || attrs[:account_id] || []).each do |i|
            @account = Account.new(i)
          end
          attrs.delete 'account_id'
          attrs.delete :account_id
        end
      end
      super(attrs)

    end
  end
end