gistfile1.rb

class Person
  include MongoMapper::Document
end

class Registree < Person
  key :in_wels,   Boolean
  key :in_vienna, Boolean
end

class Visitor
  QUERY = { "$or" => [{:in_wels => true}, {:in_vienna => true}] }
  
  def new(*args)
    r = Registree.new(*args)
    r.in_wels   = true
    r.in_vienna = true # okay maybe you don't want them both true, but...
    r
  end
  
  def find(*args)
    Registree.where(QUERY).find(*args)
  end
end