title: DBRefs vs Map/Reduce vs Manual References
body: <p>I've come across three different ways of "joining" collections:</p>
  
  <ol>
  <li>Manually keep a "foreign-key-esk" reference to the collection you wish to join with your target collection</li>
  <li>Use <a href="http://docs.mongodb.org/manual/reference/database-references/" rel="nofollow noreferrer">DBRefs</a></li>
  <li>Write a series of <a href="http://docs.mongodb.org/manual/core/map-reduce/" rel="nofollow noreferrer">Map/Reduce</a> functions to maintain the relationship</li>
  </ol>
  
  <p>Can someone explain the benefits of and when I should each one? </p>
  
  <p>My first impression is that Map/Reduce is for large, frequently used sets and the other two are mainly meant for small/fast queries.</p>