First async unit test with Mocha and Sinonjs

I m actually using a micro framework created by my society in which we use Mongoose.

To manage the mongoose object, we created a modelfactory, that returns us a model corresponding to the mongoose name object.

Actually, I m working on an authentication service in which I inject this modelfactory.

I need to unit test it with mocha and sinonjs, but I m a bit lost...

This is my authentication Service method that I want to test :

class AuthenticationService extends Service

  constructor: (modelFactory)->
    super(modelFactory)

    @authorizedClientIds = [
      "123456"
      "toto"
    ]
    @OAuthAccessTokensModel = @modelFactory.getSchema('OAuthAccessTokens')
    @OAuthClientsModel = @modelFactory.getSchema('OAuthClients')
    @OAuthUsersModel = @modelFactory.getSchema('OAuthUsers')
    @OAuthRefreshTokensModel = @modelFactory.getSchema('OAuthRefreshTokens')

  ## Get an access token from the bearer token ##
  getAccessToken: (bearerToken, callback)->
    @OAuthAccessTokensModel.findOne({accessToken: bearerToken}, callback)

module.exports = AuthenticationService

I want to test the getAccessToken method, but I have clearly no idea how to make it work...

I've tried to make something like :

describe("Authentication Service", function () {

    var service;

    before(function () {
        ModelFactory = use('/app/core/config/database/ModelFactory');

        var mock = sinon.mock(ModelFactory.getFactoryInstance([]));
        mock.expects("getSchema").withArgs("user").return({name:'user',getName:function(){}});

        service = new AuthenticationService(mock);
    });

    describe("getAccessToken", function () {
        it('should return-1 when the value is not present', function () {
            var proxy = once(service.getAccessToken());

            mock.verify();
        });
    });
});

How should I do to test it correctly ?

Thanks for advance