Laravel/Ardent - on save(), error: Relationship method must return an object of type Illuminate

I'm having some trouble getting my Laravel relationships to work out. In my application, there is a one-to-many relationship between users and ideas. (A user may have multiple ideas.) I'm using Ardent.

Here's my User model:

use Illuminate\Auth\UserTrait;

use Illuminate\Auth\UserInterface;

use Illuminate\Auth\Reminders\RemindableTrait;

use Illuminate\Auth\Reminders\RemindableInterface;



use LaravelBook\Ardent\Ardent;



class User extends Ardent implements UserInterface, RemindableInterface {



    use UserTrait, RemindableTrait;



    /**

     * The database table used by the model.

     *

     * @var string

     */

    protected $table = 'users';



    /**

     * The attributes excluded from the model's JSON form.

     *

     * @var array

     */

    protected $hidden = array('password', 'remember_token');

    protected $fillable = array('first_name', 'last_name', 'email', 'password');



    public $validation_errors;

    public $autoPurgeRedundantAttributes = true;

    public $autoHashPasswordAttributes = true;

    public $autoHydrateEntityFromInput = true;



    public static $passwordAttributes  = array('password');



    public static $rules = array(

        'first_name'            => 'required|between:1,16',

        'last_name'             => 'required|between:1,16',

        'email'                 => 'required|email|unique:users',

        'password'              => 'required|between:6,100'

    );



    public function ideas()

    {

        return $this->hasMany('Idea');

    }   

}

And here's my Idea model:

use LaravelBook\Ardent\Ardent;



class Idea extends Ardent {



    /**

     * The database table used by the model.

     *

     * @var string

     */

    protected $table = 'ideas';



    protected $fillable = array('title');



    public $validation_errors;

    public $autoPurgeRedundantAttributes = true;

    public $autoHydrateEntityFromInput = true;



    public static $rules = array(

        'title'            => 'required'

    );



    public function user()

    {

        return $this->belongsTo('User');

    }

}

Finally, here's my controller code:

class IdeasController extends BaseController {



    public function postInsert()

    {

        $idea = new Idea;



        $idea->user()->associate(Auth::user());



        if($idea->save())

        {

            return Response::json(array(

                'success' => true,

                'idea_id' => $idea->id,

                'title' => $idea->title),

                200

            );

        }

        else

        {

            return Response::json(array(

                'success' => false,

                'errors' => json_encode($idea->errors)),

                400

            );

        }

    }



}

$idea->save() throws the error:

{

  "error": {

    "type": "LogicException",

    "message": "Relationship method must return an object of type Illuminate\\Database\\Eloquent\\Relations\\Relation",

    "file": "\/var\/www\/3os\/vendor\/laravel\/framework\/src\/Illuminate\/Database\/Eloquent\/Model.php",

    "line": 2498

  }

}

At first, I was trying to set the user_id in the Idea like so:

$idea->user_id = Auth::id();

I then changed it to:

$idea->user()->associate(Auth::user());

But the results were the same.

Any suggestions would be much appreciated.