db={"game":[{_id:ObjectId("5e6099a40c0ad78d5fedd69e"),line:7,title:"Test Title",attendance:"19812",eventDate:"2020-02-22T00:30:00Z",__v:115,createdAt:"2020-03-22T22:39:47.559+00:00",updatedAt:"2020-09-08T02:09:26.430+00:00",count:65,betCount:34}],"bets":[{_id:ObjectId("5e72df1694d71f34dd561f6e"),isMatched:false,hasWon:true,user:ObjectId("5e6c5b08e1fc8ccde43b12fb"),game:ObjectId("5e6099a40c0ad78d5fedd69e"),amount:30,createdAt:"2020-03-19T02:55:18.141+00:00",updatedAt:"2020-03-23T03:04:17.062+00:00",__v:0}],"user":[{_id:ObjectId("5e6c5b08e1fc8ccde43b12fb"),name:"Jithin",bio:"My Name is Jithin.",email:"jithin@gmail.com",createdAt:"2020-03-12T21:55:49.024+00:00",updatedAt:"2020-09-09T04:59:13.769+00:00",__v:0,currentAmount:232,winnings:4}]}

db.game.aggregate([{$lookup:{from:"bets",let:{id:"$_id"},pipeline:[{$match:{$expr:{$eq:["$$id","$game"]}}},{$lookup:{from:"user",localField:"user",foreignField:"_id",as:"user"}}],as:"bets"}}])