Lazy registration with sinatra and datamapper

I have a user model in my Sinatra application that requires the user to enter an email a password and a confirmation of the password. Something along the lines of this:

class User
  include DataMapper::Resource

  property :id,                 Serial
  property :email,              String, :required => true, :unique => true, :format  => :email_address,
  property :name,               String
  property :hashed_password,    String
  property :salt,               String  
  property :created_at,         DateTime

  attr_accessor :password, :password_confirmation

  validates_presence_of     :password_confirmation
  validates_presence_of     :password
  validates_length_of       :password, :min => 6
  validates_confirmation_of :password
end

This works fine and all except I want to be able to create a user and associate other models to that user without the user entering anything but a name. After a while the user will have created a group and other users that are also associated with that group. Then I need the user to be able to enter an email address and a password and put those in the created fields in the database. I guess the validation doesn't occur until I save the user object I have created but I need to save the users in the database without the email and the password of course If they want to be able to login th开发者_StackOverflow中文版ey will need to provide the information necesarry for that.

Do I create my own validation methods or is there a way to make DataMapper validate sometimes (eg. for one post request but not for another) and sometimes not?

Check out http://datamapper.org/docs/validations and "Contextual Validations" - maybe that's what you're looking for.