Magento: Call Product Model method from view - magento

Question

I've created a custom model to override core's Product Model.
// app/code/local/Commerce121/Catalog/Model/Product.php
include('Mage/Catalog/Model/Product.php');
class Commerce121_Catalog_Model_Product extends Mage_Catalog_Model_Product
{
public function getCompatibilityGrid()
{
return '<table width="100%"><tr><th>Year</th><th>Model</th><tr>Engine</tr></tr></table>';
}
}
// app/code/local/Commerce121/Catalog/etc/config.xml
<?xml version="1.0"?>
<config>
<modules>
<Commerce121_catalog>
<version>1.0</version>
</Commerce121_catalog>
</modules>
<global>
<models>
<catalog>
<rewrite>
<product>Commerce121_Catalog_Model_Product</product>
</rewrite>
</catalog>
</models>
</global>
</config>
// app/etc/modules/Commerce121_Catalog.xml
<?xml version="1.0"?>
<config>
<modules>
<Commerce121_catalog>
<active>true</active>
<codepool>local</codepool>
</Commerce121_catalog>
</modules>
</config>
In the admin the module is listed as enabled.
In app/design/frontend/.../default/template/catalog/product/view/view.phtml
I added:
echo $_product->getCompatibilityGrid();
But nothing shows (this is the view, since echo 'bla' shows). Is $_product a reference to a controller? If so, should I need to extend the controller as well?

Answer 1

Turs out Product model wasn't overriden. There was a part missing in the config.xml which declares the module:
<commerce121_catalog>
<class>Commerce121_Catalog_Model</class>
</commerce121_catalog>
Should be like this:
<?xml version="1.0"?>
<config>
<modules>
<Commerce121_Catalog>
<version>0.1.0</version>
</Commerce121_Catalog>
</modules>
<global>
<models>
<commerce121_catalog>
<class>Commerce121_Catalog_Model</class>
</commerce121_catalog>
<catalog>
<rewrite>
<product>Commerce121_Catalog_Model_Product</product>
</rewrite>
</catalog>
</models>
</global>
</config>