How to allow only one product in a cart?

Question

We have a requirement, wherein a customer will be allowed to add only one product to the cart at a time. However he/she can add any quantity of that product.

How can I prevent a product from adding to cart if there is already a product present in the cart?

Can some one help on this?

Answer 1

The 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

<?php

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

Instead of rewriting a controller (please oh please don't do that), rather, rewrite the addProduct method to account for the limit:

class YourCompany_YourModule_Model_Cart extends Mage_Checkout_Model_Cart
{
    public function addProduct($productInfo, $requestInfo=null){
        if($this->getItemsCount()>1){
            Mage::throwException(Mage::helper('checkout')->__('Cannot add item - cart quantity would exceed checkout the limit of %s per person.', 1));
        }
        parent::addProduct($productInfo, $requestInfo);
    }
}

If you want to get fancy, replace the 1 above with Mage::getStoreConfig('checkout/options/max_cart_qty) and set the following your module's config.xml:

<default>
    <checkout>
        <options>
            <max_cart_qty>1</max_cart_qty>
        </options>
    </checkout>
</default>

That value is now controlled via the XML value. If you want to get really, really fancy, add this to the system.xml of your new module:

<config>
    <sections>
        <checkout>
            <groups>
                <options>
                    <fields>
                        <max_cart_qty translate="label">
                            <label>Maximum Quantity Allowed in Cart (total qty)</label>
                            <frontend_type>text</frontend_type>
                            <sort_order>100</sort_order>
                            <show_in_default>1</show_in_default>
                        </max_cart_qty>
                    </fields>
                </options>
            </groups>
        </checkout>
    </sections>
</config>

Remember that you need to set a <depends>Mage_Checkout</depends> to your module in order to piggyback on its' predefined system configuration.

Answer 3

A possible way is to rewrite the addAction of Mage_Checkout_CartController.

So check if there is already a product in cart and if yes show an appropriate error message. If not you can call the parent method which is doing the complete add-process:

    if (count($this->_getCart()->getProductIds()) > 0) {
        $this->_goBack();
    } else {
        parent::addAction();
    }

Answer 4

I think you can use an observer and the following events will be useful to check the conditions what you want. (Perhaps some of the events may not in the version of Magento that you are using).

checkout_cart_save_before

checkout_cart_product_add_after

checkout_cart_update_items_before

checkout_cart_update_items_after

checkout_cart_save_after

Answer 5

Maybe a bit of a left field idea but how about using the catalog_product_is_salable_after event?

What you can do here is check to see if the product is in the cart or if there is another product in the cart. When there is another product in the cart update the 'is_salable' attribute of the 'salable' object passed to the observer to false.

NOTE that this has not been tested and is only an idea. It will fail if your template does not check $product->isSaleable() before displaying the button. It would also only remove the button and not actually stop the add process if the user was able to guess the url.

Answer 6

I know this topic is something old, but i had a similar problem. I want only one item in cart and if customer adds a new one, i want to replace the old one with the new one. So i override the addAction (described here like this:

public function addAction(){
    $items = $this->_getCart()->getItems();
            foreach ($items as $item)
                    {
                            $itemId = $item->getItemId();
                            $this->_getCart()->removeItem($itemId);
                    }
    parent::addAction();

}