title: Many-to-many relationships in Google AppEngine - efficient?
body: <p>I'm using Google Appengine to store a list of favorites, linking a Facebook UserID to one or more IDs from Bing. I need function calls returning the number of users who have favorited an item, and the number of times an item has been favorited (and by whom).</p>
  
  <p>My question is, should I resolve this relationship into two tables for efficiency? If I have a table with columns for Facebook ID and Bing ID, I can easily use select queries for both of the functions above, however this will require that each row is searched in each query. The alternative is having two tables, one for each Facebook user's favorites and the other for each Bing item's favorited users, and using transactions to keep them in sync. The two tables option has the advantage of being able to use JSON or CSV in the database so that only one row needs to be fetched, and little manipulation needs to be done for an API.</p>
  
  <p>Which option is better, in terms of efficiency and minimising cost?</p>
  
  <p>Thanks,
  Matt</p>