Testing a backbone relation model by using jasmine

Let's suppose I have two simple fixture files, one for the user(1) and one for the messages(2).

The Backbone Model for the messages is the following (3).

If I load the "Message Fixture", I would like to have also the related info regarding the user as specified in Message Model.
What is the proper way to active this goal in a spec view (4) by using jasmine test suite?
Please see the comments in (4) for more details.

(1)

// User Fixture
beforeEach(function () {
    this.fixtures = _.extend(this.fixtures || {}, {
        Users: {
            valid: {
                status: 'OK',
                version: '1.0',
                response: {
                    users: [
                        {
                            id: 1,
                            name: 'olivier'
                        },
                        {
                            id: 2,
                            name: 'pierre',
                        },
                        {
                            id: 3,
                            name: 'george'
                        }
                    ]
                }
            }
        }
    });
});

(2)

// Message Fixture
beforeEach(function () {
    this.fixtures = _.extend(this.fixtures || {}, {
        Messages: {
            valid: {
                status: 'OK',
                version: '1.0',
                response: {
                    messages: [
                        {
                            sender_id: 1,
                            recipient_id: 2,
                            id: 1,
                            message: "Est inventore aliquam ipsa"
                        },
                        {
                            sender_id: 3,
                            recipient_id: 2,
                            id: 2,
                            message: "Et omnis quo perspiciatis qui"
                        }
                    ]
                }
            }
        }
    });
});

(3)

// Message model

MessageModel = Backbone.RelationalModel.extend({
    relations: [
        {
            type: Backbone.HasOne,
            key: 'recipient_user',
            keySource: 'recipient_id',
            keyDestination: 'recipient_user',
            relatedModel: UserModel
        },
        {
            type: Backbone.HasOne,
            key: 'sender_user',
            keySource: 'sender_id',
            keyDestination: 'sender_user',
            relatedModel: UserModel
        }
    ]
});

(4)

// Spec View

describe('MyView Spec', function () {
        describe('when fetching model from server', function () {
            beforeEach(function () {
                this.fixture = this.fixtures.Messages.valid;
                this.fixtureResponse = this.fixture.response.messages[0];
                this.server = sinon.fakeServer.create();
                this.server.respondWith(
                    'GET',
                    // some url
                    JSON.stringify(this.fixtureResponse)
                );
            });
            it('should the recipient_user be defined', function () {
                this.model.fetch();
                this.server.respond();
                // this.fixtureResponse.recipient_user is not defined 
                // as expected by the relation defined in (3)
                expect(this.fixtureResponse.recipient_user).toBeDefined();
            });
        });
    });
});