[
  {
    "_id": "5b506dccc00e755288ceaec6",
    "title": "Mercedes",
    "URL": "Null",
    "price": 2700000,
    "region": "Mumbai",
    "Man_year": 2013
  },
  {
    "_id": "5b506dccc00e755288ceaec6",
    "title": "Ferrari",
    "URL": "Null",
    "price": 30000,
    "region": "China",
    "Man_year": 2013
  }
]

[{"_id":{"man_year":2010},"countofcars":2,{"regions":{"Mumbai","China"}}}

db.collection('collection').aggregate([{ $group: { _id: {man_year:"$Man_year",region:"$region"}, countofcars: { $sum: 1 } } }]).toArray(function (err, results) {