[{_id:ObjectId("623e2f0cb242ee9367eb3c9f"),subjects:[{id:"1",name:"math"},{id:"2",name:"physics"}]},{_id:ObjectId("623e2f17b242ee9367eb3ca0"),subjects:[{id:"1",name:"math"},{id:"3",name:"biology"}]}]

db.collection.aggregate([{"$project":{"newSubjects":{"$arrayToObject":{"$map":{"input":"$subjects","as":"s","in":{k:"$$s.id",v:"$$s"}}}}}}])