Unclear Symfony form error "This value is already used" - forms

Question

I use Symfony 2.7.3 and I have following form ("app_cargo_source"):
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('country', 'choice', [
'choices' => $this->worldManager->getCountriesByRegionIds([1, 2]),
'choice_label' => 'name',
'choice_value' => 'id',
'label' => false,
// *this line is important* see docs for choice type
'choices_as_values' => true
])
// ...
;
// ...
}
The form is used in another form (app_cargo):
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('source', 'app_cargo_source')
// ...
;
// ...
}
“choices” field in first form is an array of WorldCountry objects, which are selected from database by regions id. Here is a picture of the array from Profiler:
WorldCountry and WorldRegion have unidirectional ManyToOne relationship.
The problem is, when I select country in country field and submit the form, back I receive following error for country field:
Why that happens, and why Symfony at all try assign a value to region?
P.S. Please let me know if more information is needed.

Answer 1

Right! It seems that problem was in uniqueness of region field in my WorldCountry entity. My WorldRegion entity is following:
/**
* WorldRegion
*
* #ORM\Table(name="world_region")
* #ORM\Entity(repositoryClass="AppBundle\Repository\WorldRegionRepository")
*
* #UniqueEntity(fields={"code"}, message="Region code must be unique")
*/
class WorldRegion
{
/**
* #var integer
*
* #ORM\Column(name="region_id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
* #Assert\NotBlank()
*
* #ORM\Column(name="region_code", type="string", length=5,
* unique = true, nullable=false)
*/
protected $code;
// ...
}
And WorldCountry:
/**
* WorldCountry
*
* #ORM\Table(name="world_country")
* #ORM\Entity(repositoryClass="AppBundle\Repository\WorldCountryRepository")
*
* #UniqueEntity(fields={"iso2"})
* #UniqueEntity(fields={"iso3"})
* #UniqueEntity(fields={"region"}) // This line should be removed!
*/
class WorldCountry
{
/**
* #var integer
*
* #ORM\Column(name="country_id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var WorldRegion
*
* #ORM\ManyToOne(targetEntity="WorldRegion",
* cascade={"all"}
* )
* #ORM\JoinColumn(name="region_id", referencedColumnName="region_id")
*/
protected $region;
// ...
}
As seen WorldCountry has unique constraint on region field. In this case it is logical error. Because ManyToOne states that in database table for WorldCountry, countries are unique, but regions - not. But I, using #UniqueEntity, claim that regions also should be unique. So, annotation #UniqueEntity(fields={"region"}) should be removed from WorldCountry entity.
P.S. Please let me know if I can improve my answer.