Laravel event cannot get data from model relationship using load() method

Question

I cannot get the data from the model with a relationship. But when I use return response()->json($chat->load('attachments')) I can get the attachments.

I want to get data Chat data with the attachments.

This is my Model Chat.php

class Chat extends Model
{
    public function attachments()
    {
        return $this->hasMany('App\ChatAttachment');
    }
}

And this is my controller Chat.php

public function storeMessage(Request $request)
{
    DB::transaction(function() use ($request) {
        $chat = new Chat;
        $chat->message = $request->message;
        $chat->save();

        event( new GetMessages($chat->load('attachments')));
    }, 2);
}

My event GetMessages.php

class GetMessages implements ShouldBroadcast
{
    use Dispatchable, InteractsWithSockets, SerializesModels;

    public $chats;
    public function __construct($chats)
    {
        $this->chats = $chats;
    }

    public function broadcastOn()
    {
      return ['get-messages'];
    }

    public function broadcastAs()
    {
      return 'get-messages';
    }
}

And my JS

Echo.channel('get-messages').listen('.get-messages', (data) => {
    console.log(data.chats) // Can see the chats object, but no attachments
    console.log(data.chats.attachments); // Undefined
})