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db.collection.aggregate([{$match:{"isLive":true,"vehicleDetails.city":{$in:["Gurugram","Delhi","New Delhi"]},"vehicleCondition":"USED"}},{"$addFields":{"cityIndex":{"$indexOfArray":[["Gurugram","Delhi","New Delhi"],"$vehicleDetails.city"]}}},{$sort:{cityIndex:1,sortPriority:1}},{$project:{cityIndex:0}}])