dm-is-mass_assignment_safe.rb

require 'dm-core'
require 'active_model'

module ActiveModel
  module MassAssignmentSecurity
    module Sanitizer
      # Returns all attributes not denied by the authorizer.
      def sanitize(attributes)
        sanitized_attributes = attributes.reject do |key, value|
          key_name = key.name rescue key
          deny?(key_name)
        end
        debug_protected_attribute_removal(attributes, sanitized_attributes)
        sanitized_attributes
      end
    end
  end
end

module DataMapper
  # = Active Model Mass-Assignment Security
  module MassAssignmentSecurity
    include ::ActiveModel::MassAssignmentSecurity

    def self.included(base)
      base.class_eval do
        class_attribute :_accessible_attributes
        class_attribute :_protected_attributes
        class_attribute :_active_authorizer
      end
      base.extend ::ActiveModel::MassAssignmentSecurity::ClassMethods
      base.extend ClassMethods
    end

    module ClassMethods

      def logger
        @logger ||= Rails.logger
      end

    end

    def attributes=(attributes, guard_protected_attributes = true)
      attributes = sanitize_for_mass_assignment(attributes) if guard_protected_attributes
      super(attributes)
    end

  end
end

user.rb

require 'dm-mass_assignment_security'

class User
  include DataMapper::Resource
  include DataMapper::MassAssignmentSecurity
  # Alternatively, you can include MassAssignmentSecurity in every DataMapper Model/Resource with this:
  # DataMapper::Model.append_inclusions(DataMapper::MassAssignmentSecurity)

  # Properties
  property :id, Serial
  property :username, String
  property :real_name, String
  property :email, String
  property :type, Discriminator

  timestamps :at

  # Mass-assignment protection
  attr_accessible :username, :real_name, :email

end