[{"temporaryshifts":[{_id:123,arr:[{_id:123321,name:"Bluh Bluh",date:"bluh bluh"}]}]},{"key":2}]

db.collection.aggregate([{"$project":{"temporaryshifts":{"$let":{"vars":{"masterKey":{"$arrayElemAt":["$temporaryshifts",0]}},"in":{"$arrayElemAt":["$$masterKey.arr",0]}},}},},])