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(...)

I think the condition is opposite: PUT should be sent when obj.id is defined.
Also I guess you assign res.data.id to data.id which is the copy of original person object, so in the second db.person.save(person).then(...) call person.id will be still undefined. But I might be wrong.

Also PUT should be something like $http.put("/person/" + obj.id, obj)

1. condition - sure, it should be opposite
2. Promise#then transforms promise value so next then call will receive modified object
3. PUT path with id - sure :)

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?