Form and availability of "formers" - laravel

Question

I am stuck on a problem that I did not think before.
I have a form formers with 3 fields (name, firstname, status).
Then, my second form is the trainings with 2 fields (date_sitting, fk_former).
Here, I have 1 sitting date -> 16/07/2019 and the former is Dujardin Luc.
Now my problem is that if I want to create a new sitting for on 18/07/2019.
How to do for that the former Dujardin Luc to be available in my dropdownlist for on 18/07/2019 or another date??
Is it, in my case, I must to create 2 files create.blade ?
The first will just have a field -> date_sitting
And the second form will have the field -> fk_former ?
Here is my TrainingController for information
public function index()
{
$trainings = Training::oldest()->paginate(5);
$formersNoTrainingToday = Training::whereDate('date_sitting', "!=", Carbon::today())
->orWhere('date_sitting', null)->get();
$formers = Former::doesntHave('trainings')->get();
return view('admin.trainings.index', compact('trainings'))
->with('i', (request()->input('page',1) -1)*5);
}

Answer 1

I can't give you the whole answer, as there are too many things to update and a few things to decide.
First, you'll need to change your index() method to both send more and different data. If a former can train on multiple days, the $formers = Former::doesntHave('trainings')->get(); is no longer valid -- you want all formers now so that you can add a new date. So just $formers = Former::get(); now.
Next, your $formersNoTrainingToday is now obsolete, as you don't want to identify those who are unavailable just today, you want a list of all training dates so compare against whatever date the user is looking for in the form. Example: If Dujardin Luc wants training on 18/07/20XX, we don't want him in a list of NoTraining because we want him available, even if he is unavailable today.
Last on the controller side, depending on how you want to do the blade side, you might want to include a variable for an AJAX date to use to send through those formers who are available on the specific date requested by the user through AJAX. For example, if you go through AJAX, you might have a method to just send those available on the date requested like this in your controller (Not exact code):
public function getFormersWithoutTrainingOnDateRequested($dateRequested){
$formersAvailableToday = Training::whereDate('date_sitting', "!=", Carbon::createFromFormat('Y-m-d', $dateRequested))
->orWhere('date_sitting', null)->get();
return json_encode($formersAvailableToday);
}
The next part is on the blade side. You have quite a few choices here, but both of the ones I recommend involve some type of JS or Jquery. In order to populate the list of formers for the day that the user requests, for best user experience, you should get them the list quickly.
Fastest would be to provide a list of trainings (date_sitting, fk_former) to the page from the index. This would be stored as a data-object on some element of the blade page. Then, when the user clicks to change the date, a JS or Jquery function would pull the list into a var, compare it against another list of former's ids stored the same way, and re-write the select list immediately, using only those formers whose ids were not matched for the date.
Second way to do this would be a little slower, but a little easier. It would involve going back to the server to get the list of formers available. So you would have an AJAX function on the same select that goes back to the server to pull those formers available on the date chosen. The success() method of the AJAX pull would provide the info to re-write the select box, or perhaps even re-make the whole form.
Sorry I can't write all the code, you'll have to fill in the blanks where you can, but this is hopefully enough to help you figure out where to go.