Limit only one item in shopping cart

Question

Is there a way to only allow one item to be added to the shopping cart and then checked out.

I tried the event + observer addition in this link: How to allow only one product in a cart?

But it doesn't seem to work with V1.9.2.3 As I am still able to add more than item to the cart and make it all the way to checkout.

Answer 1

he events catalog_product_type_prepare_full_options and catalog_product_type_prepare_lite_options are your friends

<?xml version="1.0"?>
<config>
    <modules>
        <Fooman_Example>
            <version>0.1.0</version>
        </Fooman_Example>
    </modules>
    <global>
        <models>
            <fooman_example>
                <class>Fooman_Example_Model</class>
            </fooman_example>
        </models>
        <helpers>
            <fooman_example>
                <class>Fooman_Example_Helper</class>
            </fooman_example>
        </helpers>
    </global>
    <frontend>
        <events>
            <catalog_product_type_prepare_full_options>
                <observers>
                    <fooman_example_catalog_product_type_prepare>
                        <class>Fooman_Example_Model_Observer</class>
                        <method>catalogProductTypePrepare</method>
                    </fooman_example_catalog_product_type_prepare>
                </observers>
            </catalog_product_type_prepare_full_options>
        </events>
    </frontend>
</config>

Then in your Observer class

class Fooman_Example_Model_Observer
{
    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.');
        }
    }
}

Answer 2

I don't think it's a good idea to throw error. Rather do init checkout before add product to cart will clear the old cart and create the cart with the newly added product alone. Init checkout is a child function available in mage checkout model.