Magento – Allow only one product in cart (extension)

The events catalog_product_type_prepare_full_options
and catalog_product_type_prepare_lite_options

1. create config /app/code/community/Magespy/Cart/etc/config.xml


<config>
    <modules>
        <Magespy_Cart>
            <version>0.1.0</version>
        </Magespy_Cart>
    </modules>
    <frontend>
        <events>
            <catalog_product_type_prepare_full_options>
                <observers>
                    <magespy_cart_catalog_product_type_prepare>
                        <class>Magespy_Cart_Model_Observer</class>
                        <method>catalogProductTypePrepare</method>
                    </magespy_cart_catalog_product_type_prepare>
                </observers>
            </catalog_product_type_prepare_full_options>
        </events>
    </frontend>
</config>

2.Create observer /app/code/community/Magespy/Cart/Model/Observer.php

<?php

class Magespy_Cart_Model_Observer extends Varien_Object
{
public function catalogProductTypePrepare($observer)
{
$quote = Mage::getSingleton(‘checkout/session’)->getQuote();
if($quote->getItemsCount()>=1){
Mage::throwException(‘You can only buy one product at a time.’);
}
}
}