aggregation.js

const Model = require("./model");

let agg = model.aggregate();

//match the results
agg.match({
  rated: "R"
});
//Make the projection
agg.project({
  _id: 1, //or true
  actors: 1,
  year: 1
});
//group 
agg.group({
  _id: "$year",
  /*
  //Also you can do this
  _id: {
    year: '$year',
    rated: '$rated'
  }
  */
  movieInfo:{
    $push: "$$ROOT"
  },
  total:{
    $sum: 1
  }
});

//unwind to separate the collection in objects
agg.unwind("movieInfo");

agg.exec((err, data)=>{
  console.log(data)
});

example2.js

agg.match({rated:"R"});
    agg.project({
        _id: 1,
        actors: 1,
        year: 1
    });
    agg.group({
        _id: "$year",
        actors: {
            $push: "$actors"
        },
        total: {
            $sum: 1
        }
    });
    agg.unwind('actors');
    agg.unwind('actors');
    agg.project({
        _id: 1,
        actor: "$actors",
    });
    agg.group({
        _id: "$_id",
        actors: {
            $push: "$actor"
        },
        total: {
            $sum: 1
        }
    });