[{"_id":1,"subject":"first","items":[{"item_id":"1","ratings":{"count":90,"value":"8"}},{"item_id":"2","ratings":{"count":75,"value":"9"}}]}]

db.collection.aggregate([{$addFields:{items:{$map:{input:"$items",in:{$mergeObjects:["$$this",{"ratings":{count:"$$this.ratings.count",value:{$toInt:"$$this.ratings.value"}}}]}}}}}])