title: How to disallow mix and match of products in my Magento cart for preorders
body: <p>I am selling pre-orders in my store. And for bookkeeping reasons, I cannot let customers buy both regular and pre-order products in the same order. All pre-orders have the attribute &quot;preorder&quot; set to Yes.</p>
  <p>Now I need to disallow my customers to place regular products along with pre-order products in the same shopping cart. Preferably by generating a &quot;You cannot mix regular products with pre-order products&quot; message when customers are trying to do exactly this.</p>
  <p>Any idea of how to achieve this?</p>