Mongodb aggregate $facet

Question

I'm trying to use a $facet stage of a aggregate pipeline to match and group documents, but I suspect my syntax is out.

Could anybody tell me what's wrong with this please? Keep getting the message "A Pipeline stage specification object must contain exact one field".

[
  {
   '$match': {
   'Id': 59
  }
  }, {
   '$unwind': {
   'path': '$Vehicles'
 }
 }, {

    // It's this stage that's playing up.
    '$facet': {
       'Offers': [
         {
            '$match': { 'Vehicles.VehicleGrouping': 'OOTW' }, 
            '$group': {
                '_id': '$Vehicles.Manufacturer', 
                'count': { '$sum': 1 }
            }
         }
       ]
     }
     // End of problematic stage.

   }
 ]

Thanks.

Answer 1

You have put $match and $group, in one object, try this:

db.collection.aggregate([
  {
    "$match": {
      "Id": 59
    }
  },
  {
    "$unwind": {
      "path": "$Vehicles"
    }
  },
  {
    // It's this stage that's playing up.
    "$facet": {
      "Offers": [
        {
          "$match": {
            "Vehicles.VehicleGrouping": "OOTW"
          }
        },
        {
          "$group": {
            "_id": "$Vehicles.Manufacturer",
            "count": {
              "$sum": 1
            }
          }
        }
      ]
    }  
  }
])