Laravel: eloquent orderBy hasOne relation column using with

I have a model Orders with a hasOne relation participant.

 public function participant()
    {
        return $this->hasOne('App\OrderParticipant','order_id');
    }

I need to retrieve the Orders collection sorted by participant.last_name

My approach

$orders = \App\Orders::with([
                'participant',                    
                ])
            ->where('user_id', $user->id)  
            ->orderBy('participant.last_name')
            ->get();

Fails with :

Undefined table: 7 ERROR:  missing FROM-clause entry for table \"participant\"\nLINE 1: ...1 

I've tried to sort it after collected

return $orders->sortBy('participant.last_name');

But this doesn't sort at all

BTW I'm using postgres

Thanks

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