db={post:[{"_id":ObjectId("5eeb02881982961ada625c7d"),"title":"Covid19","created_by":ObjectId("5e4e74eb380054797d9db623"),"date":1589441206774}],users:[{"_id":ObjectId("5e4d0973babf2b74ca868f4d"),"first_name":"James","last_name":"Smith","timestamp":1582106995137},{"_id":ObjectId("5e4d0973babf2b74ca868f6d"),"first_name":"Alex","last_name":"Jimy","timestamp":1582106995139}],comments:[{"_id":ObjectId("5eeb08e26fb7f270e4077617"),"text":"Nice ","post_id":ObjectId("5eeb02881982961ada625c7d"),"sender_id":ObjectId("5e4d0973babf2b74ca868f4d"),"date":1592461538923},{"_id":ObjectId("5eeb08e26fb7f270e4077619"),"text":"Good ","post_id":ObjectId("5eeb02881982961ada625c7d"),"sender_id":ObjectId("5e4d0973babf2b74ca868f6d"),"date":1592461538923}]}

db.post.aggregate([{$lookup:{from:"users",localField:"created_by",foreignField:"_id",as:"created_users"}},{$lookup:{from:"comments",let:{p_id:"$_id"},pipeline:[{$match:{$expr:{$eq:["$post_id","$$p_id"]}}}],as:"comments"}},{$lookup:{from:"users",localField:"comments.sender_id",foreignField:"_id",as:"commented_user"}},{$set:{"comments.commented_user":"$commented_user",}},{$unset:"commented_user"}])