Symfony2 validate text input when containing something only - validation

Question

I want to validate a form which holds a dropdown menu and a text input field.
The user can choose a project from the dropdown menu. If he wants to create a new project he can use the text input field next to the dropdown menu.
Here is my upload type:
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->setAction('upload')
->setMethod('POST')
// project name dropdown menu
->add('projectname', 'choice' , array(
'label' => 'upload_project_label',
'choices' => $this->projects,
'attr' => array(
'class' => 'form-control some',
'required' => 'true'
)
))
// newprojectname text input
->add('newprojectname', 'text', array(
'label' => false,
'attr' => array(
'class' => 'form-control',
'required' => false,
'placeholder' => 'upload_newprojectname_placeholder'
)
)
)
...
And this is a snippet from my upload entity:
/**
* #ORM\Column(type="text")
*
* #var string $projectname
* #Assert\NotBlank()
*/
protected $projectname;
/**
* #ORM\Column(type="text")
*
* #var string $newprojectname
* #Assert\Length(
* min = 3,
* max = 7,
* minMessage = "min message",
* maxMessage = "max message"
* )
*/
protected $newprojectname;
My question is is there a possibility of a query to check if the field newproject is set (i.e. a string is entered)? And if so let the Assert annotation do its job.

Answer 1

This can be done a number of ways, all of which will likely satisfy your requirements.
Use a custom callback - this is the quickest and most straightforward
Use an expression validator - a lot of people have issues with embedding a meta-language within PHP which is perfectly valid but this is another quick-ish way of doing things
Use group sequences, specifically the group sequence provider functionality
Which one you choose is up to you but callbacks are a quick and easy starting point that you can build on if your validation constraints become more complex.

Answer 2

This is the code block for advised solution as custom callback validation.
I had to add another function in my upload entity which looks like this:
use Symfony\Component\Validator\Context\ExecutionContextInterface;
/**
* Function holds custom validation for fields in import/upload form
* #Assert\Callback
* #param ExecutionContextInterface $context
*/
public function validate(ExecutionContextInterface $context)
{
// check new project name
$newProjectName = $this->getNewprojectname();
if(!empty($newProjectName)) {
// letters only
$pattern = '/[a-zA-Z]/';
if($this->isPatternWrong($newProjectName, $pattern)) {
$context
->buildViolation('Please use letters only.')
->atPath('newprojectname')
->addViolation();
}
// text max. 7 digits
$maxlength = 7;
if($this->isStringTooLong($newProjectName, $maxlength)) {
$context
->buildViolation('Max. length 7 digits.')
->atPath('newprojectname')
->addViolation();
}
}
}
private function isPatternWrong($string, $pattern)
{
$result = preg_match($pattern, $string);
if($result === 0) {
return true;
}
return false;
}
private function isStringTooLong($string, $length)
{
if(strlen($string) > $length) {
return true;
}
return false;
}