PHP Trait for key key-value array of various entities (e,g, Doctrine 2 ORM)

ListProvider.php

<?php
/**
 * Provide key-value list of various entities 
 *
 * Created by: Anis Ahmad <[email protected]>
 * Created at: 8/26/18 4:42 PM
 */
namespace AppBundle\Traits;

trait ListProvider
{

    /**
     * Get an array of $valueField indexed by $keyField
     *
     * @param null $keyField
     * @param null $valueField
     * @return array
     */
    public function getIndexedList($keyField = null, $valueField = null)
    {
        if (is_a($this, 'Doctrine\ORM\EntityRepository')) {
            return $this->makeEntityList($keyField, $valueField);
        }
        // else if (is_a($this, 'Other\TypeOf\Class')) {
        //    return $this->makeOtherTypeList($keyField, $valueField);
        // }

        throw new \RuntimeException("ListProvider::getList() was not implemented for ". get_parent_class($this));
    }

    /**
     * Make list of entities from a repository
     *
     * @param $keyField
     * @param $valueField
     * @return array
     */
    private function makeEntityList($keyField, $valueField)
    {
        $rawList = $this->createQueryBuilder('a')
            ->select("a.{$keyField}, a.{$valueField}")
            ->getQuery()
            ->getArrayResult();

        $list = [];
        foreach ($rawList as $item) {
            $list[$item[$keyField]] = $item[$valueField];
        }

        return $list;
    }
}