Error in my DB scheme?

$categories = $product->getMany('ProductCategory');

Fatal error: Call to undefined method modX::toArray()

    <object class="vcCategory" table="visioncart_categories" extends="xPDOSimpleObject">
    	<!-- This table holds all the categories for the products and its settings -->
    	<field key="shopid" dbtype="int" phptype="integer" precision="8" null="false" default="" />
		<field key="name" dbtype="varchar" precision="50" phptype="string" null="false" default="" />
		<field key="alias" dbtype="varchar" precision="50" phptype="string" null="false" default="" index="index" />
		<field key="description" dbtype="text" phptype="string" null="false" default="" />
		<field key="parent" dbtype="int" phptype="integer" precision="8" null="false" default="" />
		<field key="sort" dbtype="int" phptype="integer" precision="8" null="false" default="" />
		<field key="config" dbtype="text" phptype="json" null="false" default="" />
		<field key="customfields" dbtype="text" phptype="json" null="false" default="" />
		<field key="pricechange" dbtype="decimal" precision="8,2" phptype="float" null="false" default="" />
		<field key="pricepercent" dbtype="tinyint" precision="1" attributes="unsigned" phptype="boolean" null="false" default="0" />
		<field key="active" dbtype="tinyint" precision="1" attributes="unsigned" phptype="boolean" null="false" default="0" />
	
		<composite alias="ProductCategory" class="vcProductCategory" local="id" foreign="categoryid" cardinality="many" owner="local" />
		<aggregate alias="Shop" class="vcShop" local="shopid" foreign="id" cardinality="one" owner="foreign" />
    </object>
    
    <object class="vcProductCategory" table="visioncart_products_categories" extends="xPDOSimpleObject">
    	<!-- This table holds the links between categories and products -->
    	<field key="shopid" dbtype="int" phptype="integer" precision="8" null="false" default="" />
    	<field key="categoryid" dbtype="int" phptype="integer" precision="8" null="false" default="" index="index" />
    	<field key="productid" dbtype="int" phptype="integer" precision="8" null="false" default="" index="index" />
    	<field key="sort" dbtype="int" phptype="integer" precision="8" null="false" default="" />
    	
    	<aggregate alias="Product" class="vcProduct" local="productid" foreign="id" cardinality="one" owner="foreign" />
    	<aggregate alias="Shop" class="vcShop" local="shopid" foreign="id" cardinality="one" owner="foreign" />
    	<aggregate alias="Category" class="vcCategory" local="categoryid" foreign="id" cardinality="one" owner="foreign" />
    </object>
    
    <object class="vcProduct" table="visioncart_products" extends="xPDOSimpleObject">
    	<!-- This table holds all the products and its settings -->
    	<field key="shopid" dbtype="int" phptype="integer" precision="8" null="false" default="" />
    	<field key="parent" dbtype="int" phptype="integer" precision="8" null="false" default="" />
    	<field key="taxcategory" dbtype="int" phptype="integer" precision="8" null="false" default="" />
		<field key="name" dbtype="varchar" precision="100" phptype="string" null="false" default="" />
		<field key="alias" dbtype="varchar" precision="100" phptype="string" null="false" default="" index="index" />
		<field key="description" dbtype="text" phptype="string" null="false" default="" />
		<field key="articlenumber" dbtype="varchar" precision="100" phptype="string" null="false" default="" index="index" />
		<field key="price" dbtype="decimal" precision="8,2" phptype="float" null="false" default="" index="index" />
		<field key="weight" dbtype="decimal" precision="8,2" phptype="float" null="false" default="" />
		<field key="shippingprice" dbtype="decimal" precision="8,2" phptype="float" null="false" default="" />
		<field key="publishdate" dbtype="int" precision="20" phptype="timestamp" null="false" default="0" index="index" />
        <field key="unpublishdate" dbtype="int" precision="20" phptype="timestamp" null="false" default="0" index="index" />
		<field key="customfields" dbtype="text" phptype="json" null="false" default="" />
		<field key="pictures" dbtype="text" phptype="json" null="false" default="" />
		<field key="stock" dbtype="int" phptype="integer" precision="8" null="false" default="" />
		<field key="active" dbtype="tinyint" precision="1" attributes="unsigned" phptype="integer" null="false" default="0" />

		<composite alias="ProductOption" class="vcProductOption" local="id" foreign="productid" cardinality="many" owner="local" />
		<composite alias="ProductCategory" class="vcProductCategory" local="id" foreign="productid" cardinality="many" owner="local" /> 
		<aggregate alias="TaxCategory" class="vcCategory" local="taxcategory" foreign="id" cardinality="one" owner="foreign" />
		<aggregate alias="Shop" class="vcShop" local="shopid" foreign="id" cardinality="one" owner="foreign" />
    </object>

<?php
    	// Get all links to categories for this product
    	$productCategories = $product->getMany('ProductCategory');
    	
    	foreach($productCategories as $category) {
    		print_r($category->toArray());
    		exit(); // dev code
    	}

<?php
    public function calculateProductPrice($product, $asArray=false) {
    	if (is_array($product) && isset($product['id'])) {
    		$product = $this->modx->getObject('vcProduct', $product['id']);
    	}
    	
    	if ($product == null) {
    		return false;	
    	}
    	
    	if ($asArray) {
    		$productArray = array();
    	}	

    	$categoryPrices = array();
    	
    	$this->fireEvent('vcCalculateProductPrice', 'onBeforeCalculate', array(
    		'product' => &$product,
    		'asArray' => $asArray
    	));

    	// Get all links to categories for this product
    	$productCategories = $product->getMany('ProductCategory');

    	foreach($productCategories as $productCategory) {
    		$category = $productCategory->getOne('Category');
    		if ($category->active != 1) {
    			continue;	
    		}
    		
    		// Check if it's by amount
    		if ($category->get('pricechange') != '' && ($category->get('pricepercent') == '' || $category->get('pricepercent') == '0')) {
    			$priceChange = floatval($category->get('pricechange'));
    			
    			if ($priceChange > 0) {
    				$categoryPrices[$category->get('id')] = $product->get('price') + $category->get('pricechange');
    			} else {
    				$categoryPrices[$category->get('id')] = $product->get('price') - abs($category->get('pricechange'));
    			}
    		} else {
    			// Or by percentage	
    			$percentage = floatval($category->get('pricechange'));
    			
    			if ($percentage > 0) {
    				$percentagePrice = ($product->get('price') / 100) * $percentage;
    				$categoryPrices[$category->get('id')] = $product->get('price') + $percentagePrice;	
    			} else {
    				$percentagePrice = ($product->get('price') / 100) * abs($percentage);
    				$categoryPrices[$category->get('id')] = $product->get('price') - $percentagePrice;
    			}
    		}
    	}
    	    	
    	// Get the cheapest price
    	$cheapest = min($categoryPrices);
    	
    	$product->set('price', $cheapest);
    	
    	$this->fireEvent('vcCalculateProductPrice', 'onAfterCalculate', array(
    		'product' => &$product,
    		'asArray' => $asArray
    	));

    	// Get the tax category
    	$taxCategory = $product->getOne('TaxCategory');
    	$taxAmount = ($product->get('price') / 100) * abs($taxCategory->get('pricechange'));
    	$totalAmount = number_format(($product->get('price') + $taxAmount), 2, '.', '');
    	
    	if ($asArray) {
    		$productArray['ex'] = $product->get('price');
    		$productArray['in'] = $totalAmount;
    		return $productArray;
    	}
    	
    	return $totalAmount;
    }