gistfile1.js

db = {
    person: {
        save: function(obj){
            if(obj.id){
                return $http.post("/person", obj).then(function(res){ 
                    var data = angular.extend({}, obj); // create copy of `obj` + `id` field
                    data.id = res.data.id;
                    return data;
                })
            } else {
                return $http.put("/person", obj)
            }
        }
    }
}

var person = {firstName: 'John', lastName: 'Smith'}
db.person.save(person).then(...)
person.firstName = "Michael"
db.person.save(person).then(...)

Ad 2. But it doesn't mean your person object will be modified, right? You would have to modify it in then callback:

.then(function(success){
    person = success.data;
});

Do I miss anything?