Good or bad? Extending site_content with a custom table

$content .= $this->get("price");

<?xml version="1.0" encoding="UTF-8"?>
<model package="ugallerymeta" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM"> 
    <object class="ugalleryMeta" table="site_content" extends="modResource">
        <composite alias="ugalleryMeta" local="id" foreign="ugallery" cardinality="one" owner="local" />
    </object>
 	
	<object class="ugallery_metadata" table="ugallery_metadata" extends="xPDOSimpleObject">
        <field key="ugallery" dbtype="int" phptype="integer" null="false" default="0" index="index" />
		<field key="price" dbtype="int" precision="11" phptype="integer" null="false" />
		<field key="status" dbtype="tinytext" phptype="string" null="false" />
		<field key="technic" dbtype="varchar" precision="255" phptype="string" null="false" />
		<field key="meta_data" dbtype="text" phptype="string" null="false" />
                ... a lot more fields here ...
		<aggregate alias="ugalleryMetadata" local="ugallery" foreign="id" cardinality="one" owner="foreign" />
	</object>
	
</model>

<?php
$obj = $modx->getObject('MyObject', $c);
$price = $obj->get('price');

$oUG = $modx->getObject("ugallery_metadata", array());
var_dump($oUG->get("price"));

// filename: ugallerymeta.class.php

require_once MODX_CORE_PATH.'model/modx/modprocessor.class.php';
require_once MODX_CORE_PATH.'model/modx/processors/resource/create.class.php';
require_once MODX_CORE_PATH.'model/modx/processors/resource/update.class.php';

class ugalleryMeta extends modResource {
    
	public $showInContextMenu = true;
    
    function __construct(xPDO & $xpdo) {
        parent :: __construct($xpdo);
        $this->set('class_key','ugallerymeta');
		#echo "<pre>".print_r($this,true)."</pre>";
		#die();
    }

    public static function getControllerPath(xPDO &$modx) {
        return $modx->getOption('ugallerymeta.core_path',null,$modx->getOption('core_path').'components/ugallerymeta/').'controllers/';
    }

    public function getContextMenuText() {
        $this->xpdo->lexicon->load('ugallerymeta:default');
        return array(
            'text_create' => $this->xpdo->lexicon('ugallerymeta'),
            'text_create_here' => $this->xpdo->lexicon('ugallerymeta_create_here'),
        );
    }

    public function getResourceTypeName() {
        $this->xpdo->lexicon->load('ugallerymeta:default');
        return $this->xpdo->lexicon('ugallerymeta');
    }

    public function getContent(array $options = array()) {
		$content = parent::getContent($options);
        $year = date('Y');
        $content .= '<div class="copyright">© '.$year.'. All Rights Reserved. - KEKS</div>';
		$content .= $this->get("price");
        return $content;
    }
}


class ugalleryMetaCreateProcessor extends modResourceCreateProcessor {
}

class ugalleryMetaUpdateProcessor extends modResourceUpdateProcessor {
    /**
     * Do any processing before the fields are set
     * @return boolean
     */
    public function beforeSet() {
        $beforeSet = parent::beforeSet();
        /**
        if ($this->getProperty('shouldBeTrue') == false) {
          $this->addFieldError('fieldname','The error message');
        }
        */
        return $beforeSet;
    }

    /**
     * Do any processing before the save of the Resource but after fields are set.
     * @return boolean
     */
    public function beforeSave() {
        $beforeSave = parent::beforeSave();
        if ($this->object->get('longtitle') == 'Send an Error') {
            $this->addFieldError('longtitle','Specify a different longtitle!');
        }
		$this->object->set('content', "Replaced Content"); // works fine
		$this->object->set('price', "123"); // no effect
        return $beforeSave;
    }

    /**
     * Do any custom after save processing
     * @return boolean
     */
    public function afterSave() {
        $afterSave = parent::afterSave();
        $this->modx->log(modX::LOG_LEVEL_DEBUG,'Saving a Copyrighted Page!');
        return $afterSave;
    }	
}