title: MongoDB allowDiskUse not working..
body: <p>Experts. </p>
  
  <p>I'm new to MongoDB, but know enough to get my self in trouble.. case in point: </p>
  
  <pre><code>db.test.aggregate(
  [
      {$group: {_id: {email: "$email", gender: "$gender"}, cnt: {$sum: 1}}}, 
      {$group: {_id: "$_id.email", cnt: {$sum: 1}}}, 
      {$match: {cnt: 2}}
  ], 
      {allowDiskUse : true}
  )
  </code></pre>
  
  <p>and no matter what variations I try, I keep getting the same error ("Pass allowDiskUse:true to opt in"): </p>
  
  <blockquote>
    <p>Error("Printing Stack Trace")@:0 ()@src/mongo/shell/utils.js:37
    ([object Array],[object Object])@src/mongo/shell/collection.js:866
    @(shell):7</p>
    
    <p>uncaught exception: aggregate failed: {   "errmsg" : "exception:
    Received error in response from mongo1.mscnet.com:27017: { $err:
    \"Exceeded memory limit for $group, <strong>but didn't allow external sort.
    Pass allowDiskUse:true to opt in.</strong>\", code: 16945 }",    "code" : 16945,
      "ok" : 0
        }</p>
  </blockquote>
  
  <p>FYI: I'm using Mongo 2.6</p>