Laravel resources - how to check if given value exists?

Question

I'm using laravel resources to get data for api:

        return [
            'id' => $this->id,
            'unread' => $this->unread,
            'contact' => UserResource::collection($this->users),
        ];    

And this is working fine. The problem is when 'users' is empty. I mean - not every time my controller is loading users from my relations:

    $offers = Offer::where('user_id', $user_id)->orderBy('created_at','desc')->get();
    foreach ($offers as $offer)
    {
        $offer->setRelation('users', $offer->users->unique());
    }
    return OfferShortResource::collection($offers);

sometimes - this relation does not exists. But not the relation is problem, because - my resource is not loading the relation - the relatin is loaded inside controller.

So how can I add some kind of logic to resources? - to tell basically - users property may not exists - then do not load data here, or even - do not check $this->users relation

edit: I tried to make it like this:

 use Illuminate\Support\Arr;
 ...

      'contact' => $this->when(Arr::exists($this, 'users'), function () {
                    return UserResource::collection($this->users);
                }),

but this is giving always false

edit 2: relation in the Offer model

 public function users(){
        return $this->belongsToMany(User::class, 'messages', 'offer_id', 'from')
       ->where('users.id', '!=', auth()->user()->id);

Answer 1

Try using whereHas:

Offer::whereHas('users', function ($query) use ($user_id) {
    $query->where('user_id', $user_id);
})->orderByDesc('created_at')->get();

The optional helper is useful for accessing properties that may not be defined:

optional($this->user)->name

The when method should work for you, try the following:

'contact' => $this->when(property_exists($this, 'users'), function () {
    return UserResource::collection($this->users);
}),

Also try:

isset($this->users)