Check for unique constraint violation

I have 2 fields on my entity that form a unique constraint: fieldA and fieldB, mapped to database columns field_a and field_b, respectively.

My input filter requires both of the fields:

public function init()
{
    parent::init();

    $this->add([
        'name' => 'field_a',
        'required' => true,
        'allow_empty' => false,
    ]);

    $this->add([
        'name' => 'field_b',
        'required' => true,
        'allow_empty' => false,
    ]);
}

I am trying to figure out the best way to validate that those 2 fields are unique in the database table. The input filter will fail validation if there is already a different entity with those same field values.

I was thinking I would override the isValid function and put my custom logic in there.

I would suggest to use callback validator ( Zend\Validator\Callback ) on both fields and put your custom logic in the callback function.

I would use InputFilter to add filters and validators to the form fields but you can implement InputFilterProviderInterface directly in the Form/Fieldset class.

use Zend\InputFilter\InputFilter;

class FormFilter extends InputFilter
{

    public function __construct()
    {
       $this->add(
        array(
            'name' => 'field_a',
            'filters' => array(),
            'validators' => array (
                array(
                    'name' => 'Zend\Validator\Callback',
                    'options' => array(
                        'messages' => array(
                                \Zend\Validator\Callback::INVALID_VALUE => 'Custom Message',
                        ),
                        'callback' => array($this,'validateFieldA'),
                    ),
                ), 

            )
        )
       );

      $this->add(
        array(
            'name' => 'field_b',
            'filters' => array(),
            'validators' => array (
                array(
                    'name' => 'Zend\Validator\Callback',
                    'options' => array(
                        'messages' => array(
                                \Zend\Validator\Callback::INVALID_VALUE => 'Custom Message',
                        ),
                        'callback' => array($this,'validateFieldB'),
                    ),
                ), 

            )
        )
       );

    }

    public function validateFieldA($value, $context)
    {
        // $value contains the field_a value
        // $context['field_b'] contains the field_b value

        // put your custom logic here
        // return true if the fields are unique 
        // return false if the fields are not unique
    }

    public function validateFieldB($value, $context)
    {
          ....
    }


}