Add a observation (remark) after a sitting of training

I have in fact 3 forms:

The first is students with 2 fields (name, firstname).

The second is trainings with 2 fields (date_sitting, fk_student)`.

The next is the form observations with 3 fields (title, description, fk_student).

My problem is that I can add 2 dates sittings for 1 observation for the same date.

Is it it's possible to make a blocking for this genre of case?

Here is an idea of my code so far:

public function store(Request $request)
    {
        $request->validate([
                'title' => 'required|string|max:30',
                'description' => 'required|string|max:80',
                'fk_student' => 'required'

        ]);

        $exists = Observation::where('title', $request->get('title'))->where('description', $request->get('description'))->where('fk_student', $request->get('fk_student'))->count();

        if (!$exists){

        $sittings = Training::where('fk_student', $request->get('fk_student'))->first();


        if(!isset($sittings)){ // No payment = block creation and return error msg.
            return redirect()->route('observations.index')
                    ->with('error', 'No sitting, no observation for you!');
        }
        else{
            Observation::create($request->all());
            return redirect()->route('observations.index')
                    ->with('success', 'new data created successfully');
            }
        }

        if(isset($exists)){
            return redirect()->route('observations.index')
                    ->with('error', 'Duplicate ! ');
        }
    }

Thank you for your help.

via Chebli Mohamed