Angular unit test ngResource $save method on instance object

I have a code which instantiate a new object from $resource Api.User = $resource(...blahblah...) that I'm using as :

'use strict';

angular
  .module('Exo.users')
  .controller('UsersCreateCtrl', function (collection, Api, FormRemoteErrors, $modalInstance) {
    var vm = this;

    vm.user = new Api.user({ name: '' });

    vm.save = function (isValid) {
      vm.submitted = true;
      if (isValid) {
        vm.saving = true;
        vm.user.$save().then(function (user) {
          collection.push(user);
          $modalInstance.close(true);
        }, function (errors) {
          vm.saving = false;
        });
      }
    };
  });

As you can see, there is a save function in which I'm calling vm.user.$save in order to save the new user. I need some help in order to be able to test it.

In my specs, I first try to mock the Api.User object along with the $save function :

beforeEach(module('Exo.users', function ($provide) {
  Api = {
    User: jasmine.createSpy('User').and.returnValue(function () {
      name: '',
      $save: jasmine.createSpy()
    });
  }
  $provide.value('Api', Api);
}));

it('create a new user with an empty name', function () {
  expect(scope.user.name).toEqual('');
});

it('calls the $save method on user instance', function () {
  scope.save(true);
  expect(scope.user.$save).toHaveBeenCalled();
});

Unfortunately, I've got the following error :

TypeError: 'undefined' is not an object (evaluating 'vm.user.$save().then')

I understand that I need to mock the then function but how to do it and how to also test what happen when the promise succeed or fails? I know that this question has been asked already but somehow I cannot find any relevant answers.