Meteor - Mongo aggregate does not have $count stage [duplicate]

Question

I am using meteorhacks:aggregate package to do Mongo aggregation in Meteor. I want to get the count at the last stage of the pipeline so I use this code:

Message.aggregate([
  {
    $match: {
      // ...
    }
  }, {
    $count: 'count'
  }
]);

It is pretty simple and should work, but I only get this error:

Exception while invoking method 'methodname' 
MongoError: Unrecognized pipeline stage name: '$count'
...

Please help, thanks.

Updated: this question is not duplicated as an editor suggested, my main intention is to find out why I can not use $count

Answer 1

$count is available in mongodb version 3.4. For previous versions, you will need to use $group over a constant field.

Message.aggregate([
  {
    $match: {
      // ...
    }
  }, {
    $group: {
      _id : null, 
      count : {$sum : 1}
    }
  }
]);