Custom canonical tags

Question

I am trying to override Magento's addLinkRel method to set custom canonical tags: I want my shop's canonical tags to point to the main shop's pages.

So I created code/local/Fkoessler/MyProduct/etc/config.xml:

<config>
    <modules>
        <Fkoessler_MyProduct>
            <version>0.1</version>
        </Fkoessler_MyProduct>
    </modules>
    <global>
        <blocks>
            <page>
                <rewrite>
                    <html_head>Fkoessler_MyProduct_Block_Html_Head</html_head>
                </rewrite>
            </page>
        </blocks>
    </global>
</config>

And code/local/Fkoessler/MyProduct/Block/Html/Head.php:

class Fkoessler_MyProduct_Block_Html_Head extends Mage_Page_Block_Html_Head
{
    /**
     * Add Link element to HEAD entity
     *
     * @param string $rel forward link types
     * @param string $href URI for linked resource
     * @return Mage_Page_Block_Html_Head
     */
    public function addLinkRel($rel, $href)
    {
        $href = str_replace('/paris', '', $href);
        $href = str_replace('/nimes', '', $href);
        $href = str_replace('/niort', '', $href);
        $this->addItem('link_rel', $href, 'rel="' . $rel . '"');
        return $this;
    }
}

However, my custom method is not picked up at all. What am I doing wrong?

---

EDIT:

So I understand that using an observer is better than using class inheritance. Here is how I set it up:

etc/modules/Fkoessler/Canonical.xml
<?xml version="1.0"?>
<config>
    <modules>
        <Fkoessler_Canonical>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
              <Aitoc_Aitsys/>
            </depends>
        </Fkoessler_Canonical>
    </modules>
</config>

-

app/code/local/Fkoessler/Canonical/etc/config.xml
<?xml version="1.0"?>
<config>
    <modules>
        <Fkoessler_Canonical>
            <version>0.1</version>
        </Fkoessler_Canonical>
    </modules>
    <global>
        <models>
            <fkoessler_canonical>
                <class>Fkoessler_Canonical_Model</class>
            </fkoessler_canonical>
        </models>
        <events>
            <core_block_abstract_to_html_before>
                <observers>
                    <fkoessler_catalog_core_block_abtract_to_html_before>
                        <type>singleton</type>
                        <class>fkoessler_canonical/observer</class>
                        <method>coreBlockAbstractToHtmlBefore</method>
                    </fkoessler_catalog_core_block_abtract_to_html_before>
                </observers>
            </core_block_abstract_to_html_before>
        </events>
    </global>
</config>

-

app/code/local/Fkoessler/Canonical/Model/Observer.php
class Fkoessler_Canonical_Model_Observer
{

    public function coreBlockAbstractToHtmlBefore($observer)
    {
        error_log('im observing you');
        $block = $observer->getBlock();
        if ($block instanceof Mage_Page_Block_Html_Head) {
//            $block->removeItem('link_rel', [the current URL]);
//            $block->addLinkRel('canonical', [edited URL]);
        }
    }
}

So I managed to enter my coreBlockAbstractToHtmlBefore method when the core_block_abstract_to_html_before is fired. Now how can I get the canonical url set by Magento core to modify and return it?

$observer->getBlock(); returns an instance of Mage_Page_Block_Html

Answer 1

You can use an observer to reset they value.

config.xml - add an observer

<core_block_abstract_to_html_before>
    <observers>
        <[namespace]_[module]_core_block_abstract_to_html_before>
            <type>singleton</type>
            <class>[Namespace]_[Module]_Model_Observer</class>
            <method>coreBlockAbstractToHtmlBefore</method>
        </[namespace]_[module]_core_block_abstract_to_html_before>
    </observers>
</core_block_abstract_to_html_before>

observer.php

public function coreBlockAbstractToHtmlBefore($o)
{
    $block = $o->getBlock();
    if($block instanceof Mage_Page_Block_Html_Head) {
        $url = Mage::helper('core/url')->getCurrentUrl();
        if (Mage::registry('current_product')) {
           $url = Mage::registry('current_product')->getProductUrl();
        } else if (Mage::registry('current_category')) {
           $url = Mage::registry('current_category')->getUrl();
        }
        $block->removeItem('link_rel', $url);
        // do your magic with the $url var here
        $block->addLinkRel('canonical', $url);
    }
}