Answered Mapping to Data to Related Table with create processor

"categories":["1","2","3"]

class PositionCreateProcessor extends modObjectCreateProcessor {
    public $classKey = 'rtPositions';
    public $languageTopics = array('recroot:default');
    public $objectType = 'recroot.position';

    public function beforeSave() {
        $now = new DateTime();
        $this->object->set('createdon',$now->format('Y-m-d h:i:s'));
        $title = $this->getProperty('title');
        $company = $this->getProperty('company');
        $type = $this->getProperty('type');
        $source = $this->getProperty('source');
  	$applylink = $this->getProperty('applylink');
  	$categories = $this->getProperty('categories');
        $description = $this->getProperty('description');
        $expireson = $this->getProperty('expireson');
        if (empty($title)) {$this->addFieldError('title',$this->modx->lexicon('recroot.position_err_ns_title'));}
        if (empty($company)) {$this->addFieldError('company',$this->modx->lexicon('recroot.position_err_ns_company'));}
        if (empty($type)) {$this->addFieldError('type',$this->modx->lexicon('recroot.position_err_ns_type'));}
        if (empty($source)) {$this->addFieldError('source',$this->modx->lexicon('recroot.position_err_ns_source'));}
        if (empty($applylink)) {$this->addFieldError('applylink',$this->modx->lexicon('recroot.position_err_ns_applylink'));}
        if (empty($categories)) {$this->addFieldError('categories',$this->modx->lexicon('recroot.position_err_ns_categories'));}
        if (empty($description)) {$this->addFieldError('description',$this->modx->lexicon('recroot.position_err_ns_description'));}
        if (empty($expireson)) {$this->addFieldError('expireson',$this->modx->lexicon('recroot.position_err_ns_expireson'));}
        return parent::beforeSave();
    }
    
    public function afterSave() {
        $position_id = $this->object->get('id');
        foreach ($categories as $category) { <-- dying here as $categories is undefined
            $newMap = $xpdo->newObject('rtPositionCategoriesMap',array('position_id' => $position_id,'category_id' => $category,));
            $newMap->save();
            $this->modx->log(modX::LOG_LEVEL_DEBUG, 'Mapped Category ' . $category . ' to position ' . $position_id); 
        }
        return true;
    }
}
return 'PositionCreateProcessor';