Get availability of formers for a sitting

I have 2 forms, the first is the form formers with 2 fields (name, firstname).

I also have the form trainings with 2 fields (date_sitting, fk_former).

My problem, if I want to add an other sitting today (07/07/2019), I would like to see only the formers who have no training today.

Here, I retrieve a former who has a sitting today...

Do you think it's possible to get only the formers who have no of sitting for now? It's annoying

In my Controller I have this:

Public function store(Request $request)
    {
        $request->validate([
                'date_sitting' => 'required|date',
                'fk_former' => 'required'

        ]);

        $exists = Training::where('date_sitting', $request->get('date_sitting'))->where('fk_former', $request->get('fk_former'))->count();

        if (!$exists){
            Training::create($request->all());
            return redirect()->route('trainings.index')
                ->with('success', 'new data created successfully');
        }

        else{
            return redirect()->route('trainings.index')
                ->with('error', 'duplicata');

        }   
    }

And in my create.blade.php I have this:

<div class="form-group ">
 <label for="company-content">Name & Firstname </label>
  <select name="fk_former" id="fk_former" class="form-control" required="required" value=""/>
     <option value="">Choice former</option>
          @foreach($formers as $former)
     <option value=""  >
        
      </option>
         @endforeach
{!! $errors->first('fk_former', '<span class="help-block">:message</span>') !!}
 </select>

via Chebli Mohamed