How redirect simple products of Configurable product

Question

How to redirect a simple product's page to the parent configurable product page?

Answer 1

the code below should work only if the simple products are assigned to only one configurable product.
by default, magento allows you to add a simple product to as many configurable products you need.

Let's create a module and call it StackExchange_Redirect.
You will need the following files.

app/etc/modules/StackExchange_Redirect.xml - the declaration file

<?xml version="1.0"?>
<config>
    <modules>
        <StackExchange_Redirect>
            <codePool>local</codePool>
            <active>true</active>
            <depends>
                <Mage_Catalog />
            </depends>
        </StackExchange_Redirect>
    </modules>
</config>

app/code/local/StackExchange/Redirect/etc/config.xml - the module configuration file where you declare an observer for the product view page

<?xml version="1.0"?>
<config>
    <modules>
        <StackExchange_Redirect>
            <version>1.0.0</version>
        </StackExchange_Redirect>
    </modules>
    <global>
        <models>
            <stackexchange_redirect>
                <class>StackExchange_Redirect_Model</class>
            </stackexchange_redirect>
        </models>
    </global>
    <frontend>
        <events>
            <controller_action_predispatch_catalog_product_view>
                <observers>
                    <stackexchange_redirect>
                        <class>stackexchange_redirect/observer</class>
                        <method>redirectToConfigurable</method>
                    </stackexchange_redirect>
                </observers>
            </controller_action_predispatch_catalog_product_view>
        </events>
    </frontend>
</config>

app/code/local/StackExchange/Redirect/Model/Observer.php - the observer that should redirect to the configurable product page.

<?php 
class StackExchange_Redirect_Model_Observer
{
    public function redirectToConfigurable($observer)
    {
        $id = Mage::app()->getRequest()->getParam('id');
        $configurableParentIds = Mage::getResourceSingleton('catalog/product_type_configurable')
            ->getParentIdsByChild($id);
        if (isset($configurableParentIds[0])) {
            $mainProduct = Mage::getModel('catalog/product')
                ->setStoreId(Mage::app()->getStore()->getId())
                ->load($configurableParentIds[0]);
            Mage::app()->getResponse()->setRedirect($mainProduct->getProductUrl(), 301);
            Mage::app()->getResponse()->sendResponse();
            exit;
        }
        return $this;
    }
}

when you are done, clear the cache.