db={"blogs":[{"_id":ObjectId("60e09c90402cbd625d7a8162"),"title":"...","author":ObjectId("60ce9146f41866120ee70c0d"),"posts":[]},{"_id":ObjectId("60e09c97402cbd625d7a8163"),"title":"...","author":ObjectId("60ce9146f41866120ee70c0d"),"posts":[]},{"_id":ObjectId("60e09d06402cbd625d7a8164"),"title":"...","author":ObjectId("60ce9146f41866120ee70c0d"),"posts":[]},{"_id":ObjectId("60e09d07402cbd625d7a8165"),"title":"...","author":ObjectId("60ce9146f41866120ee70c0d"),"posts":[{"_id":ObjectId("60e12300e931cd14c03ecd89"),"title":"...","author":ObjectId("60ce9146f41866120ee70c0d")},{"_id":ObjectId("60e12305e931cd14c03ecd8a"),"title":"...","author":ObjectId("60ce9146f41866120ee70c0d")}]}],"users":[{"_id":ObjectId("60ce9146f41866120ee70c0d"),"name":"Rahul kumar","status":"A Fake Developer","__v":0,"pic":"https://res.cloudinary.com/bdevg/image/upload/v1604287418/pic_xexz8o.jpg"}]}

db.blogs.aggregate([{$unwind:{path:"$posts",preserveNullAndEmptyArrays:true}},{"$lookup":{"from":"users","localField":"posts.author","foreignField":"_id","as":"posts.author"}},{$unwind:{path:"$posts.author",preserveNullAndEmptyArrays:true}},{$group:{_id:"$_id",title:{$first:"$title"},posts:{$push:"$posts"},author:{$first:"$author"}}},{"$lookup":{"from":"users","localField":"author","foreignField":"_id","as":"author"}},{$unwind:{path:"$author",preserveNullAndEmptyArrays:true}},])