title: How to do HAVING COUNT in MongoDB?
body: <p>My documents look like this:</p>
  
  <pre><code>{
      "_id": ObjectId("5698fcb5585b2de0120eba31"),
      "id": "26125242313",
      "parent_id": "26125241841",
      "link_id": "10024080",
      "name": "26125242313",
      "author": "gigaquack",
      "body": "blogging = creative writing",
      "subreddit_id": "6",
      "subreddit": "reddit.com",
      "score": "27",
      "created_utc": "2007-10-22 18:39:31"
  }
  </code></pre>
  
  <p>What I'm trying to do is create a query that finds users who posted to only 1 subreddit. I did this in SQL by using the query:</p>
  
  <pre><code>Select distinct author, subreddit from reddit group by author having count(*) = 1;
  </code></pre>
  
  <p>I'm trying to do something similar in MongoDB but are having some troubles atm.
  I managed to recreate select distinct by using aggregate group but I can't figure out how to solve the HAVING COUNT part.</p>
  
  <p>This is what my query looks like:</p>
  
  <pre><code>db.collection.aggregate( 
  [{"$group": 
      { "_id": { author: "$author", subreddit: "$subreddit" } } },
      {$match:{count:1}} // This part is not working
  ])
  </code></pre>
  
  <p>Am I using $match wrong?</p>