[{"source":"Chennai","destination":"Kolkata","flights":[{"flightId":"E6-2145","seatsAvailable":10,"fare":[{"travelClass":"Economy","baseFare":3588},{"travelClass":"Business","baseFare":9999}]}]}]

db.collection.aggregate([{$set:{flights:{$map:{input:"$flights",as:"flight",in:{$mergeObjects:["$$flight",{fare:{$map:{input:"$$flight.fare",in:{$cond:{if:{$and:[{$eq:["$$this.travelClass","Economy"]},{$lt:["$$this.baseFare",3800]}]},then:{$mergeObjects:["$$this",{baseFare:3800}]},else:"$$this"}}}}}]}}}}}])