[{_id:ObjectId("625ebcabcc03685c273b2363"),reviews:[{by:{type:ObjectId("625ebcabcc03685c273b2364"),ref:"User",},title:{type:"dfdf",},message:{type:"sdf",},rating:7,imagesUri:["sdf","sdfd"],timestamp:ISODate("2022-04-19T13:44:11.781Z")},{by:{type:ObjectId("625ebcabcc03685c273b2365"),ref:"User",},title:{type:"dfdf",},message:{type:"sdf",},rating:5,imagesUri:["sdf","sdfd"],timestamp:ISODate("2022-03-19T13:44:11.781Z")}]},]

db.collection.aggregate([{"$addFields":{ratingSum:{$reduce:{initialValue:0,input:"$reviews",in:{$sum:["$$value","$$this.rating"]}}}}},{"$addFields":{"rating":{"$divide":["$ratingSum",{$size:"$reviews"}]}}}])