Difference between .save and $save to resource in angularjs

Question

I've seen code that both calls $save and save to a $resource in angular.

What is the difference and when do you use either?

Answer 1

Best explanation === example :

// by writing '{ id: '@id' }' we want the id to be taken from 'id' parameter in request data, hence the '@' sign. Note that this mechanism is available for non-GET RQs only:
var Notes = $resource('/notes/:id', { id: '@id' });

var noteId = "my_note1";
// below we specify 'id' explicitly - has to be done for GET RQ:
// operations on our note are done inside callback function, just to make sure that the note is resolved:
var note = Notes.get({ id: noteId }, function () {

    // let's make some changes:
    note.topic = "A brand new topic here!";

    // save using $resource "static" action (aka "class" action). 'id' is taken from data object:
    Notes.save(note);
    // We can overwrite 'id' just like this: 
    Notes.save({ id: "some_other_noteId" }, note);

    // even more changes:
    note.body = "Blah blah blah, new boring body is here";

    // this time save using instance action. Again: 'id' is taken from data object:
    note.$save();
    // changing id with instance action? there you go:
    note.$save({ id: "yet_another_noteId" });

    // Naturally, we could just:
    note.id = "OMG_how_many_of_those_noteIds_has_he_left";
    Notes.save(note);
    // ... and with instance action:
    note.id = "OK_he_wins";
    note.$save();
});

Even custom $resource actions (defined by you) have their $-prefixed counterparts, as long as they're non-GET - see http://docs.angularjs.org/api/ngResource.$resource#example_creating-a-custom-put-request.
And no, not all actions have instance method version. What would be the point of invoking GET on an instance? From official ngResource docs:

The action methods on the class object or instance object can be invoked with the following parameters:

• HTTP GET "class" actions: Resource.action([parameters], [success], [error])
• non-GET "class" actions: Resource.action([parameters], postData, [success], [error])
• non-GET instance actions: instance.$action([parameters], [success], [error])

Answer 2

$save is a method added by an action of $resource. save can be a method by special resource.

all actions have $ prefixed methods. read more here: http://docs.angularjs.org/api/ngResource.$resource