title: Generate radio buttons on NM relation using form builder on symfony formbuider
body: <p>When I add this field this way</p>
  <pre><code>-&gt;add('options', EntityType::class, [
                  'class' =&gt; Disease::class,
                  'query_builder' =&gt; function (EntityRepository $er) use ($options) {
                      $qb = $er-&gt;createQueryBuilder('g');
                      $qb
                          -&gt;where('g.activeIn LIKE :product')
                          -&gt;andWhere('g.name IN(:names)')
                          -&gt;setParameter('product', '%&quot;'.$options['product']-&gt;getId().'&quot;%')
                          -&gt;setParameter('names', ['option1','option2'])
                          -&gt;orderBy('g.category');
                      return $qb;
                  },
                  'expanded' =&gt; true,
                  'multiple' =&gt; true,
                  'required' =&gt; false,
                  'disabled' =&gt; $readOnly,
                  'label_attr' =&gt; [
                      'class' =&gt; 'font-weight-bold'
                  ]
                  ])
  </code></pre>
  <p>but it generates 2 checkboxes . Itry to set<code>multiple</code> as false in order to get radio buttons but it generates an error</p>
  <pre><code>Entity of type &quot;Doctrine\ORM\PersistentCollection&quot; passed to the choice field must be managed. Maybe you forget to persist it in the entity manager?
  </code></pre>
  <p>There is any way to render radio buttons at the form , in a n:m relationship as I have in this case?</p>