title: How to use RelatedFieldWidgetWrapper in django 3
body: <p>What should I put in rel and admin site?
  This is the field in my form:</p>
  <pre class="lang-py prettyprint-override"><code>tutor_temporal = forms.ModelChoiceField(
      queryset=Tutor_temporal.objects.all(),
      label='Tutor No Registrado', 
      required=False,
      widget=RelatedFieldWidgetWrapper(
          widget=forms.Select(attrs={'class': 'input is-small is-rounded'}),
          rel=Tutor_temporal._meta.get_field('id').rel,
          admin_site= admin_site
      )
  )
  </code></pre>
  <p>The problem is that when I try that, throws this AttributeError: 'AutoField' object has no attribute 'rel', because apparently is deprecated.</p>