[{"userId":100,rating:2.3,comment:"comment sss"},{"userId":100,rating:1.5,comment:"comment sss"},{"userId":110,rating:3.3,comment:"comment sss"},{"userId":100,rating:6,comment:"comment sss"},{"userId":100,rating:3.3,comment:"comment sss"},{"userId":110,rating:2.3,comment:"comment sss"},]

db.collection.aggregate([{"$group":{"_id":"$userId","ratings":{$push:"$rating"}}},{"$project":{"_id":false,"userId":"$_id","avgRating":{"$avg":"$ratings"}}}])