title: MongoDB - simulate join or subquery
body: <p>I'm trying to figure out the best way to structure my data in Mongo to simulate what would be a simple join or subquery in SQL.</p>
  
  <p>Say I have the classic Users and Posts example, with Users in one collection and Posts in another. I want to find all posts by users who's city is "london". </p>
  
  <p>I've simplified things in this question, in my real world scenario storing Posts as an array in the User document won't work as I have 1,000's of "posts" per user constantly inserting.</p>
  
  <p>Can Mongos $in operator help here? Can $in handle an array of 10,000,000 entries?</p>