atomic_set_in_mongo.rb

@collection = db.collection('docs')

loop do 
  # Find a record to work on
  new_record = @collection.find_one({:state => "NEW"}, {:sort => "created_at asc"})

  # Set the state to my process id, but only if it's still in the "NEW" state
  response = @collection.update({:_id => new_record.id, :state => "NEW"},
                                {'$set' => {:state => Process.pid}}, 
                                {:upsert => false, :safe => true})

  if response[0][0][:n] == 0  # Don't ask...
    # We updated no records, so someone else got to this one while we were trying to lock it.  Move on.
    puts "Decorator #{@pid}: Failed to lock record #{mention.id}; skipping"
    sleep 1
    next 
  end
  
  # do some work, possibly modifying new_record

  @collection.update({:_id => new_record.id}, new_record.merge(:state => "DONE"))

end