[{"_id":"12","title":"Vanella Icream","contain":"sugar","details":[{"flavour":"Vanella"},{"weight":"6KG"}]},{"_id":"3","title":"Pretzels","contain":"salt","details":[{"flavour":"Wheat"},{"weight":"2KG"}]}]

db.collection.aggregate({"$group":{"_id":null,"details":{"$addToSet":""}}})