Check the number of seance before the encoding of a training

In my form Payments, I have 4 fields (date_payment, type_sitting, number_seance, fk_student)

Then, in my form trainings I have 3 fields (date_sitting, fk_payment, fk_student).

I want to do a checking in my form trainings and block with an error message if the number of seance is unlimited.

Here is an idea of my Training Controller

public function store(Request $request)
    {
        $request->validate([
                'date_sitting' => 'required',
                'fk_payment' => 'required',
                'fk_student' => 'required'


        ]);


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

       if (!$exists){


        $payment = Payment::where('fk_student', $request->get('fk_student'))->first();


        if(!isset($payment)){ 
            return redirect()->route('trainings.index')
                    ->with('error', 'No Payment, no training for you!');
        }
        else{
            Training::create($request->all());
            return redirect()->route('trainings.index')
                    ->with('success', 'new data created successfully');
        }
    }


    }

via Chebli Mohamed