How to add condition in add-to-cart button function? Magento 1.9

Question

How to Add PHP script in add-to-cart function like if other products are trying to add add-to-cart, supposed selected id based products[restricted products] already available in add-to-cart here restrict and show message like not allowed to add-to-cart, please purchase alone, if already some other products available in cart condition like you need to purchase this product alone.

Using Magento add-to-cart button Function how to achieve my above condition?

Answer 1

Create an event observer for event sales_quote_merge_before. Create a custom module and put this in config.xml.

<events>
    <sales_quote_merge_before>
        <observers>
            <cart_validation>
                <class>MyModule_CartValidation_Model_Observer</class>
                <method>quoteMergeBefore</method>
            </cart_update>
        </observers>
    </sales_quote_merge_before>

</events>

Create Observer.php in [MyModule]/[CartValidation]/Model and put this code.

class MyModule_CartValidation_Model_Observer {
    public function quoteMergeBefore($observer) {
        // Get Existing Cart Items
        $quoteItem = Mage::getSingleton('checkout/cart') - > getQuote() - > getItemsCollection();

        //Get current product trying to be added
        $product = Mage::getModel('catalog/product')
            - > load(Mage::app() - > getRequest() - > getParam('product', 0));

        /*
        *  Your code logic to see restricted items in cart
        *  
        */
        $isRestricted = $this - > hasRestrictedItems($quoteItem, $product);
        if ($isRestricted) {
            Mage::getSingleton("core/session") - > addError("You can not add this product");
            return;
        }

    }
}

Make sure you implement your restricted products logic in hasRestrictedItems().