Assume we have this entity:

class Author
{
    /**
     * @Assert\Collection(fields={
     *     "alternate_email" = @Optional({@Assert\Email})
     * })
    public $profileData = array();
}

The annotations and PHP format is working as expected. However, the Yaml and Xml format isn't working. On the documentation, we (ryan, @afishnamedsquish and I) have put some time in trying to get a working Yaml and Xml format, but it seems like it's not possible.

If we have this:

Acme\AuthorBundle\Entity\Author:
    properties:
        profileData:
             - Collection:
                fields:
                    alternate_email:
                        - Collection\Optional:
                            - Email: ~

We get this error:

Fatal error: Class 'Symfony\Component\Validator\Constraints\Collection\OptionalValidator'

It's going wrong in Collection line 50:

if (!$field instanceof Optional && !$field instanceof Required) {
    $this->fields[$fieldName] = $field = new Required($field);
}

$field is an instance of Optional or Required when using PHP or annotations. When using Yaml, $field is an array:

array (size=1)
  0 => 
    object(Symfony\Component\Validator\Constraints\Collection\Optional)[4300]
      public 'constraints' => 
        array (size=1)
          0 => 
            object(Symfony\Component\Validator\Constraints\Email)[4301]
              ...
      public 'groups' => 
        array (size=1)
          0 => string 'Default' (length=7)

Because it's not an instance of Optional or Required, it becomes:

object(Symfony\Component\Validator\Constraints\Collection\Required)[4302]
  public 'constraints' => 
    array (size=1)
      0 => 
        object(Symfony\Component\Validator\Constraints\Collection\Optional)[4300]
          public 'constraints' => 
            array (size=1)
              ...
          public 'groups' => 
            array (size=1)
              ...
  public 'groups' => 
    array (size=1)
      0 => string 'Default' (length=7)

And that's when we are in trouble, because the ValidatorFactory will search for OptionalValidator.

I have tried every Yaml syntax I know to make $field a class and not an array, but I can't find it. I think some change in the YamlFileLoader is needed to make these edge cases work.

This is the same for Xml:

<?xml version="1.0" encoding="UTF-8" ?>
<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping http://symfony.com/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">

    <class name="Acme\AuthorBundle\Entity\Author">
        <property name="profileData">
            <constraint name="Collection">
                <option name="fields">
                    <value key="alternate_email">
                        <constraint name="Collection\Optional">
                            <constraint name="Email" />
                        </constraint>
                    </value>
                </option>
            </constraint>
        </property>
    </class>
</constraint-mapping>

If you want some background on this issue, see symfony/symfony-docs#2432 and this gist.