[{"_id":"12","title":"Vanella Icream","contain":"sugar","details":[{"flavour":"Vanella"},{"weight":"15KG"}]},{"_id":"123","title":"Pretzels","contain":"salt","details":[{"flavour":"Wheat"},{"weight":"15KG"}]},{"_id":"44","title":"Blabama","contain":"sweet","details":[{"flavour":"Wheat"},{"weight":"2KG"}]},{"_id":"3","title":"Carnival","contain":"Sweet","details":[{"flavour":"Vanella"},{"weight":"15KG"}]}]

db.collection.aggregate([{$project:{_id:0,details:1}},{$unwind:"$details"},{$project:{details:{$objectToArray:"$details"}}},{$unwind:"$details"},{$group:{_id:"$details.k",v:{$addToSet:"$details.v"}}},{$group:{_id:null,details:{$push:{k:"$_id",v:"$v"}}}},{$project:{_id:0,details:{$arrayToObject:"$details"}}}])