Minimum total of items in shopping cart to allow checkout - magento

Question

I'm trying to find a configuration that allows one of my customer groups (wholesale) to add any given number of items to their shopping carts, but have a restricted checkout equal or greater than 16 items in total.
For example:
8 items from product A
2 items from product B
6 items from product C
That would be a total of 16 items and it would be possible for them to checkout.
I tried configuring the Minimum Qty Allowed in Shopping Cart, but then, they have to get 16 items of each product.
Do you know if there is a way to configure, add an extension or hardcode something to solve this problem?
Thank you for your time!

Answer 1

Easiest way is to set a minimum order amount
/admin/system_config/edit/section/sales
To implement a minimum item restriction, you can place an observer to fire on one page checkout which checks the item quantity it cart and redirects you back to the cart if its below your threshold with a message.
Full prototype, flavor as needed:
app\etc\modules\Spirit_Cms.xml
<?xml version="1.0"?>
<config>
<modules>
<Spirit_Cms>
<active>true</active>
<codePool>local</codePool>
</Spirit_Cms>
</modules>
</config>
app\code\local\Spirit\Cms\etc\config.xml
<?xml version="1.0"?>
<config>
<modules>
<Spirit_Cms>
<version>0.0.1</version>
</Spirit_Cms>
</modules>
<frontend>
<events>
<controller_action_predispatch_checkout_onepage_index>
<observers>
<spirit_cms_restrict_checkout>
<class>Spirit_Cms_Model_Observer</class>
<method>restrictCheckout</method>
</spirit_cms_restrict_checkout>
</observers>
</controller_action_predispatch_checkout_onepage_index>
</events>
</frontend>
<global>
<models>
<spirit_cms>
<class>Spirit_Cms_Model</class>
</spirit_cms>
</models>
</global>
</config>
app\code\local\Spirit\Cms\Model\Observer.php
<?php
class Spirit_Cms_Model_Observer
{
public function restrictCheckout( $oObserver )
{
// Ensure we only observe once.
if( Mage::registry( 'restrict_checkout_flag' ) )
{
return $this;
}
else
{
$oQuote = Mage::getSingleton( 'checkout/cart' )->getQuote();
$oCartItems = $oQuote->getAllItems();
$iTotalQty = 0;
foreach( $oCartItems as $oCartItem )
{
$iTotalQty = $iTotalQty + $oCartItem->getQty();
}
if( $iTotalQty < 12 )
{
$oSession = Mage::getSingleton( 'checkout/session' );
$oSession->addError( 'Please add at least 12 items to your cart.' );
Mage::app()->getResponse()->setRedirect( Mage::getUrl( 'checkout/cart' ) );
}
Mage::register( 'restrict_checkout_flag', 1, TRUE );
}
}
}
?>