composition.rb

class UserFinder
  def self.find(id)
    record = User.find(id)
    "#{record.user_type}User".constantize.new(record)
  end
end

class User < ActiveRecord::Base
end

class LDAPUser < SimpleDelegator
  def foo
    common_data + ldap_specific_data
  end
end

class LocalUser < SimpleDelegator
  def foo
    common_data + local_specific_data
  end
end

UserFinder.find(23)