db.mongodb.aggregate(
    [
        {
            "$group" : {
                "_id" : {
                    "post_meta᎐country" : "$post_meta.country"
                },
                "COUNT(_id)" : {
                    "$sum" : NumberInt(1)
                }
            }
        },
        {
            "$sort" : {
                "COUNT(_id)" : NumberInt(-1)
            }
        },
        {$limit: 10}
    ]
);

aggregation <- mongodb_connection$aggregate(
  '[{
  "$group" : {"_id":"$post_meta.country",
  "count": {"$sum":1}
  }
  }]
  '
)

Error: Invalid JSON object

aggregation <- mongodb_connection$aggregate(

      '[{
      "$group" : {"_id":"$post_meta.country",
      "count": {"$sum":1}
      },
      {
      "$sort": {"count":-1}
      }]
      '
      )