[{"firstName":"Shaun","salary":[{"id":1,"rate":250,},{"id":2,"rate":290,}]},{"firstName":"Julian","salary":[{"id":1,"rate":750,},{"id":2,"rate":760,},{"id":3,"rate":790,},]}]

db.collection.aggregate({$project:{firstName:1,rate:{$arrayElemAt:["$salary.rate",-1]}}})