[{"name":"Natural Product","ratings":[{"user":"5fdce4bd75dbe4864fcd5001","rating":5},{"user":"5fdce4bd75dbe4864fcd5002","rating":4}]}]

db.collection.aggregate([{$match:{_id:{$exists:true}}},{$unwind:"$ratings"},{$group:{_id:"$_id",rating:{$avg:"$ratings.rating"}}}])