title: mongo db crosss collection query
body: <p>I have two collections in MongoDB, one is placements and the other is tids.</p>
  
  <p>Placements collection has program_id, placement_id, tids collection has placement_id and tid_id. </p>
  
  <p>I want be able to find all placements with program_id as 3,5 and then find all the tid_id in tids collection with those placement_id, so in SQL it would be:</p>
  
  <pre><code>select tids.tid_id 
  from tids 
  inner join placements 
  where tids.placement_id = placements.placement_id
  and placements.program_id = 3 or 5
  </code></pre>
  
  <p>It says MongoDB doesn't support join, so is it even possible to do a query like this? </p>