title: Validating Multiselect CakePHP 3 belongsToMany Entites
body: <p>I'm want to ensure that a multi-select field (belongsToMany) has at least one item checked/selected. I have two entities:</p>
  
  <p>Listings and PaymentTypes - belongsToMany.</p>
  
  <p>My field is created with the helper:</p>
  
  <pre><code>echo $this-&gt;Form-&gt;input('payment_types._ids', ['options' =&gt; $paymentTypes, 'multiple' =&gt; 'checkbox']);
  </code></pre>
  
  <p>In my ListingsModel.php file I have my validator setup as follows:</p>
  
  <pre><code>public function validationDefault(Validator $validator) {
  
          $validator = new Validator();
  
          $validator-&gt;requirePresence('payment_types._ids');
  
          return $validator;
  
  }
  </code></pre>
  
  <p>This prevent the save from happening but I don't see any validation error message. Why doesn't the HTML5 form validation work as it does on other fields which are of type text?</p>