title: Trying to populate in mongoose only if ref is not null - not working
body: <p>I'm trying to get a list of books with their author information.</p>
  
  <p>Some users were deleted and therefore they have not longer a document in the db, therefore, their info is null.</p>
  
  <p>I'm trying to pull books ONLY if their creators still exist. </p>
  
  <p>This is my code:</p>
  
  <pre><code>  Book.find({_creator:{$ne:null}}).populate(
          {
              path: '_creator',
              match: { _id: { $ne: null }}
          })
          .exec(function (err,books) {
          if(err) throw err;
          if(books) {
              res.send(books)
          }
      })
  </code></pre>
  
  <p>This is what it returns:</p>
  
  <pre><code>  [
      {
          "_id":"55d98e6a4de71010099c59eb",
          "dateOfCreation":"2015-08-23T09:12:10.095Z",
          "_creator":null,
          "__v":0,
          "coverUrl":"cover14403211323926quv.png",
          "description":"asdasd",
          "name":"asdasd"
      }
  ]
  </code></pre>
  
  <p>Notice that the _creator field IS null. 
  Why is that? </p>