access_token_validation.rb

require 'mongo_mapper'

MongoMapper.database = 'tmp'

class HasValidAccessTokensValidator < ActiveModel::EachValidator
  VALID_ACCESS_TOKENS = ['read', 'write', 'admin']
  
  def validate_each(record, attribute, value)
    unless value.all? { |token| VALID_ACCESS_TOKENS.include?(token) }
      record.errors[attribute] << (options[:message] || "has invalid access_tokens")
    end
  end
end

class User
  include MongoMapper::Document
  key :access_tokens, Array
  
  validates :access_tokens, :has_valid_access_tokens => true
end

p User.new(:access_tokens => ['read']).valid?
p User.new(:access_tokens => ['read','write']).valid?
p User.new(:access_tokens => ['read', 'write', 'super-duper-admin']).valid?

# For more information on ActiveModel validations: 
# http://edgeguides.rubyonrails.org/3_0_release_notes.html#validations