user.rb

class User
  include Chassis::Persistence

  attr_accessor :name, :phone_number, :token, :device

  def push?
    !!device.push_token
  end

  def save
    raise "Users must have tokens!" unless token
    super
  end
end

user_repo.rb

class UserRepo
  # Delegate operations to the default Repo with the correct
  # object_class
  extend Chassis::Repo::Delegation

  class UnknownTokenError < StandardError
    def initialize(token)
      @token = token
    end

    def to_s
      "Could not identifiy user with token: #{@token}"
    end
  end

  class UnknownPhoneNumber < StandardError
    def initialize(phone_number)
      @phone_number = phone_number
    end

    def to_s
      "Could not identifiy user with phone number: #{@phone_number}"
    end
  end

  # Define application specific methods used to access the objects
  # in a meaningful way
  class << self
    def find_by_token!(token)
      user = query UserWithToken.new(token)
      raise UnknownTokenError, token if user.nil?
      user
    end

    def find_by_phone_number!(phone_number)
      user = query UserWithPhoneNumber.new(phone_number)
      raise UnknownPhoneNumber, phone_number if user.nil?
      user
    end
  end
end

# Define selectors which are passed when querying the repo
UserWithToken = Struct.new :token
UserWithPhoneNumber = Struct.new :phone_number

@inem sorry about! The code was taken from a project using an older version of chassis. Chassis::HashInitializer was in an older version. It's been replaced with Chassis::Initializable which uses a block and hash. Chassis::Persistence now includes Chassis::Initializable. Serialization was a custom module I wrote for that project. It should not have been in the example.