adding constraint to fields

Export/Export/ListPerson.php

@@ -13,6 +13,8 @@ use Chill\MainBundle\Export\FormatterInterface;
 use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
 use Symfony\Component\Translation\TranslatorInterface;
+use Symfony\Component\Validator\Constraints\Callback;
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
 
 /**
  * Render a list of peoples
@@ -59,7 +61,16 @@ class ListPerson implements ListInterface
             'multiple' => true,
             'expanded' => true,
             'choices' => array_combine($this->fields, $this->fields),
-            'label'    => 'Fields to include in export'
+            'label'    => 'Fields to include in export',
+            'constraints' => [new Callback(array(
+                'callback' => function($selected, ExecutionContextInterface $context) {
+                    if (count($selected) === 0) {
+                        $context->buildViolation('You must select at least one element')
+                            ->atPath('fields')
+                            ->addViolation();
+                    }
+                }
+            ))]
         ));
 
     }