ZF2 - inputFilter -> set value to null - mysql

Question

In my ZF2 app I've got a form with several optional fields. If user decide to leave that field blank then it should set it as NULL in db, however - it doesn't work.
Example field:
$this->add(array(
'name' => 'productId',
'type' => 'select',
'options' => array(
'value_options' => array(
'global' => 'Global option', // this should be null
'mobile' => 'Another option'
)
),
));
And filter:
$inputFilter->add(array(
'name' => 'productId',
'validators' => array(
array(
'name' => 'Callback',
'options' => array(
// 'messages' => array(
// \Zend\Validator\Callback::INVALID_VALUE => $value,
// ),
'callback' => function ($value) {
if($value === 'global')
{
$value = null;
//return true;
// echo '<pre>' . var_export($value, true) . '</pre>';
// die();
}
}
),
),
),
));
This piece of code works when "mobile" option is selected. However, with 'global' option it doesn't work.
As you can see, I've done some basic debugging to make sure that value is overrided on callback and it indeed returns NULL. Regardless of that validator says:
array (
'productId' =>
array (
'callbackValue' => 'The input is not valid',
),
)
So I tried to return true on callback which resulted in below error:
Statement could not be executed (23000 - 1452 - Cannot add or update a child row: a foreign key constraint fails (app.codes, CONSTRAINT fk_products_id FOREIGN KEY (productId) REFERENCES products (id)))
How am I supposed to pass null param to my db? If I cut this field completly from form then it is ignored and everything works.

Answer 1

Found solution, it's pretty simple actually.
I can set desired select options to some specific value and then use ToNull filter to convert it into null -
https://framework.zend.com/manual/2.4/en/modules/zend.filter.null.html
Code for $inputFilter:
$inputFilter->add(array(
'name' => 'productId',
'required' => false,
'filters' => array(
array('name' => 'ToNull', 'options' => array('type' => \Zend\Filter\ToNull::TYPE_STRING)),
),
));