db={orders:[{"_id":"wJNEiSYwBd5ozGtLX","orderId":52713,"createdAt":ISODate("2020-01-31T04:34:13.790Z"),"status":"closed","orders":[{"_id":"ziPzwLuZrz9MNkaRT","productId":10290,"quantity":2},{"_id":"ziPzwLuZrz9MNkaRY","productId":10291,"quantity":2}]}],products:[{"_id":"238cwwLkZa6gKNN86","productId":10290,"title":"Product Title","price":9.9},{"_id":"238cwwLkZa6gKNN87","productId":10291,"title":"Product Title","price":9}]}

db.orders.aggregate([{$unwind:"$orders"},{"$lookup":{"from":"products","localField":"orders.productId","foreignField":"productId","as":"products"}},{"$set":{"orders.price":{"$arrayElemAt":["$products.price",0]}}},{$group:{_id:"$_id",createdAt:{$first:"$createdAt"},status:{$first:"$status"},orderId:{$first:"$orderId"},orders:{$push:"$orders"}}}])