Cannot Extend Laravel Model

I have 2 models. The User model, and the relationship works correctly, I use tinker, and I can see the application that is associated with the user.

User::find(4)->application

However, the application will not return the user - in tinker I get null, whats worse, if I try to access rep in tinker, I get Bad Method Exception Call

Application::find(8)->user

is null

Note: I have an id column in users which I "find" users. and there is "ucid" column in users that I have defined as the primaryKey in Application.

Application Model:

class Application extends Model
{
    protected $data = [
        'data' => 'array'
    ];
    protected $primaryKey = 'ucid';
    protected $fillable = [
        'ucid', 'data'
    ];
    public function user() 
    {
        return $this->belongsTo(User::class,'ucid');
    }
    public function rep() 
    {
        return 'Test';
    }
}

User Model

class User extends Authenticatable
{
    public function application()
    {
        return $this->hasOne(Application::class,'ucid');
    }
}

What am I missing?

via Chebli Mohamed