gistfile1.rb

require 'mongo_mapper'
include MongoMapper

MongoMapper.database = "mm-test"
$db = MongoMapper.database

class EncryptedPassword
  def self.to_mongo(value) 
    print "to_mongo(#{value.inspect})...";
    return('') if value.nil?
    puts(value == 'plain text' ? 'OK' : "Error? #{caller[0..3]}")
    'encrypted text'
  end
  def self.from_mongo(value)
    print "from_mongo(#{value.inspect})...";
    return('') if value.nil?
    puts(value == 'encrypted text' ? 'OK' : "Error? #{caller[0..3]}")
    'plain text'
  end
end

class LoginDetails
  include MongoMapper::EmbeddedDocument
  key :login,    String
  key :password, EncryptedPassword
end

class User1
  include MongoMapper::Document
  key :name, String
  key :login, LoginDetails
end
class User2
  include MongoMapper::Document
  key :name, String
  key :password, EncryptedPassword
end

User1.collection.remove
User2.collection.remove

@l=LoginDetails.new; @l.password = 'plain text'
puts "\nExpect errors here...\n\n"
@u1=User1.create(:login => @l)
puts "\nbut none here...\n\n"
@u2=User2.create(:password => 'plain text')