¿Cómo poner registros seleccionados en un "Multiselect" de un formulario de edición?

I have a form that registers a "sample" with your "essays".

BD: sample, essay, essay_sample

Models: Sample, Essay, EssayShow

The registration form works very well, the problem is that when I want to retrieve the "essays" of each sample entered in an edition form of that sample, I can not recover them.

<div class="form-group">
  {!! Form::Label('pedido', 'Ensayos:') !!}
    <select class="chosen-select" name="pedido[]" multiple>
      @foreach($ensayos as $ensayo)
        <option value="" @if($muestra->ensayos()->find($muestra_id)) selected @endif> / </option>
      @endforeach
    </select>
</div>

How can I solve this problem? From already thank you very much

via Chebli Mohamed