Entity Field Query and Entity Metadata Wrapper example: Updates the stock field on products from orders with a past a defined order id (in pending status). I had to write this because the stock rules became corrupt after a series of updates to the site.

Update product stock from order history

<?php
$order_id = 10;

// Create the entity field query object.
$efq = new EntityFieldQuery();

// Add the required filters to the query and execute it.
$result = $efq->entityCondition('entity_type', 'commerce_order')
->propertyCondition('order_id', $order_id, '>=')
->propertyCondition('status', 'pending')
->execute();

if (!empty($result['commerce_order'])) {

  // Loop through each of the results returned.
  foreach ($result['commerce_order'] as $oid) {
    $order = commerce_order_load($oid->order_id);
    
    // Create a metadata wrapper for easy read/write operations on the entites.
    $order_wrapper = entity_metadata_wrapper('commerce_order', $order);
    
    // Loop through the commerce line items.
    foreach ($order_wrapper->commerce_line_items as $line_item_wrapper) {
      $line_item = $line_item_wrapper->value();
      $product = $line_item_wrapper->commerce_product->value();
      
      // Proceed only if the product contains a stock field.
      if (!empty($product->commerce_stock)) {

        // Obtain the stock using the wrapper.    
        $stock = $line_item_wrapper->commerce_product->commerce_stock->value();
        if ($stock > 0) {
          $update = ($stock - $line_item->quantity);
          
          // Set the updated stock using the wrapper.
          $line_item_wrapper->commerce_product->commerce_stock->set($update);
          commerce_product_save($product);
        }
      }
    }
  }
}

?>