Database
[{"_id":"5fe43eb44cd6820963c98c32","name":"Monday Diet","userID":"5f225d7458b48d0fe897662e","day":"Monday","type":"private","mealList":[{"_id":ObjectId("5fe43eb44cd6820963c98c33"),"time":"Breakfast","foodList":[{"_id":"5fe43eb44cd6820963c98c34","foodName":"Eggs","Qty":"2","calories":"calories","category":"category"}]},{"_id":"5fe43eb44cd6820963c98c36","time":"Lunch","foodList":[{"_id":"5fe43eb44cd6820963c98c37","foodName":"food1","Qty":"100g"},]}],"createdAt":"2020-12-24T07:09:40.141Z","updatedAt":"2020-12-24T07:09:40.141Z","__v":0}]
Query
db.collection.update({"mealList._id":ObjectId("5fe43eb44cd6820963c98c33")},{$push:{"mealList.$.foodList":{"_id":"5fe43eb44cd6820963c98c35","foodName":"demo","Qty":"3","calories":"calories","category":"category"}}})