Symfony - How to apply validation based on user input using validation groups

In Symfony applications validation constraints are applied to the Entity and not to the Form. In some cases, however, you'll need to validate an object against only some constraints on that class. To do this, you can organize each constraint into one or more "validation groups", and then apply validation against just one group of constraints.

http://symfony.com/doc/current/book/validation.html#book-validation-validation-groups

Even better, you can determine which validation group should be applied based on the value filled in form by user:

http://symfony.com/doc/current/book/forms.html#groups-based-on-the-submitted-data

Let's say we have a   Product entity. Product entity has a 2 properties: description and category.
Depending on the "category "property different validation groups can be applied to 'description'. Symfony defines a group called "Default" in which are included all the validation which are not marked as part of any group.

<?php

class Product
{
    /**
     * @var string
     *
     * @ORM\Column(name="description", type="text",  nullable=false)
     * @Assert\NotNull(groups={"imported"})
     */
    protected $description;

    protected $category;
}

I marked the NotNull validation as being part of "imported" group.

Now in the form class using the Product entity I need to put the logic based on which the validation groups are used:

<?php

use AppBundle\Entity\Product;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class ProductType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
       ....
    }
    // ...
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'validation_groups' => function (FormInterface $form) {
                $data = $form->getData();

                if ($data->getCategory() === 'Exotic fruits' ) {
                    return array('Default', 'imported');
                }

                return array('Default');
                },
        ));
    }
}

The code above basically says :
 - if category is "Exotic fruits" than apply "Default" and "imported" validations.
- otherwise apply only "Default" validations

Sometimes you need advanced logic to determine the validation groups. If they can't be determined by a simple callback, you can use a service:   http://symfony.com/doc/current/cookbook/validation/group_service_resolver.html