Writing custom validation rules by extending existing one (3.X) Urgent help Needed!

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$rules->add($rules->isUnique(['item_id', 'manufacture_unit_id'], 'This Item-Unit Already Inserted.'));

By this we can check uniqueness of current insertion.

But I need to extend it and want to add more checking on it. So I’ve written this and included it .

use App\Model\Rule\CustomIsUnique;

   $rules->add(new CustomIsUnique(['item_id', 'manufacture_unit_id']), [
        'errorField' => 'item_id',
        'message' => 'Item Unit must be unique.'
    ]);

How to extend the existing isUnique method?
Inside my CustomIsUnique rule I just copied IsUnique’s Code.

<?php

namespace App\Model\Rule;

use Cake\Datasource\EntityInterface;

class CustomIsUnique
{
/**
* The list of fields to check
*
* @var array
*/
protected $_fields;

/**
 * The options to use.
 *
 * @var array
 */
protected $_options;

/**
 * Constructor.
 *
 * ### Options
 *
 * - `allowMultipleNulls` Set to false to disallow multiple null values in
 *   multi-column unique rules. By default this is `true` to emulate how SQL UNIQUE
 *   keys work.
 *
 * @param array $fields The list of fields to check uniqueness for
 * @param array $options The additional options for this rule.
 */
public function __construct(array $fields, array $options = [])
{
    $this->_fields = $fields;
    $this->_options = $options + ['allowMultipleNulls' => true];
}

/**
 * Performs the uniqueness check
 *
 * @param \Cake\Datasource\EntityInterface $entity The entity from where to extract the fields
 *   where the `repository` key is required.
 * @param array $options Options passed to the check,
 * @return bool
 */
public function __invoke(EntityInterface $entity, array $options)
{
    if (!$entity->extract($this->_fields, true)) {
        return true;
    }
    $allowMultipleNulls = $this->_options['allowMultipleNulls'];
    $alias = $options['repository']->alias();
    //  pr($alias);die;
    $conditions = $this->_alias($alias, $entity->extract($this->_fields), $allowMultipleNulls);

    if ($entity->isNew() === false) {
        $keys = (array)$options['repository']->primaryKey();
        $keys = $this->_alias($alias, $entity->extract($keys), $allowMultipleNulls);
        if (array_filter($keys, 'strlen')) {
            $conditions['NOT'] = $keys;
        }
    }
  //  pr($conditions);
   // pr(!$options['repository']->exists($conditions));die;
    return !$options['repository']->exists($conditions);
}

/**
 * Add a model alias to all the keys in a set of conditions.
 *
 * Null values will be omitted from the generated conditions,
 * as SQL UNIQUE indexes treat `NULL != NULL`
 *
 * @param string $alias The alias to add.
 * @param array $conditions The conditions to alias.
 * @param bool $multipleNulls Whether or not to allow multiple nulls.
 * @return array
 */
protected function _alias($alias, $conditions, $multipleNulls)
{
    $aliased = [];
    foreach ($conditions as $key => $value) {
        if ($multipleNulls) {
            $aliased["$alias.$key"] = $value;
        } else {
            $aliased["$alias.$key IS"] = $value;
        }
    }

    return $aliased;
}

}