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

This should have been happen. And it happen.
In chassis you have PersistEnce, but here you typed PersistAnce
Also, there's no such thing like HashInitializer in Chassis. Did you meant Initializable?

And where did Serialization came from? ActiveModel::Serialization?

@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.