gistfile1.rbx

class MonitoredTopic
  include DataMapper::Resource

  property :id, Serial
  
  belongs_to :user
  belongs_to :topic  
end

class Topic
  include DataMapper::Resource
  include DataMapper::Timestamp

  has n, :monitored_topics
  has n, :monitors, :through => :monitored_topics, :class_name => 'User'
end

class User
  include DataMapper::Resource
  include DataMapper::Validate
  include DataMapper::Timestamp

  has n, :monitored_topics
end