contact.rb

class Contact < Person
end

gistfile1.txt

>> Person.find_or_create_by_external('email', '[email protected]') { |p, id| p.becomes Contact }
  MOPED: localhost:27017 QUERY        database=delight_development collection=people selector={"identifiers.key"=>"[email protected]", "identifiers._type"=>"UserIdentifier::Email"} flags=[:slave_ok] limit=-1 skip=0 fields=nil (0.0ms)
  MOPED: localhost:27017 INSERT       database=delight_development collection=people documents=[{"_id"=>4fe37df00420ba6116000006, "_type"=>"Contact", "updated_at"=>2012-06-21 20:02:56 UTC, "created_at"=>2012-06-21 20:02:56 UTC, "identifiers"=>[{"_id"=>4fe37df00420ba6116000007, "_type"=>"UserIdentifier::Email", "verified"=>false, "key"=>"[email protected]", "key_hash"=>"6dcf8f85513186d6f8cd8d3274f6eeaae22adb85"}]}] flags=[] (0.0ms)
=> #<Person _id: 4fe37df00420ba6116000006, _type: "Contact", created_at: 2012-06-21 20:02:56 UTC, updated_at: 2012-06-21 20:02:56 UTC, first_name: nil, last_name: nil>

person.rb

class << Person
  def find_or_initialize_by_external type, key
    unless user = external(type,key).first
      user = new
      identifier = user.identifiers.build({},UserIdentifier.type_to_class(type))
      identifier.key = key
      yield(user,identifier) if block_given?
    end
    user
  end

  def find_or_create_by_external type, key, &blk
    find_or_initialize_by_external(type,key,&blk).tap {|u| u.save}
  end
end