WordPress – Combining orders and merging duplicate line items in Woocommerce

I’m writing a code for Woocommerce that will automatically combine all the orders that come in from the same customer until we flip the status. The only problem I’m having is that line items are being duplicated when a customer places a second order containing one of the same items as on a previous order. This makes picking orders more confusing for our staff.

I have included a section that checks to see if the item exists in the previous order, and it then updates the quantity of that item instead of adding a new line. This part of the code is causing an error at checkout: "Error processing checkout. Please try later." Without this part of the code, everything works as expected.

// Add a custom function to check and combine orders
function combine_orders_on_status_change($order_id) {
    // Get the current order
    $current_order = wc_get_order($order_id);

    // Check if the order status is 'on-hold'
    if ($current_order->get_status() === 'on-hold') {
        // Get user ID of the current order
        $user_id = $current_order->get_user_id();

        // Get all orders of the same user with status 'on-hold'
        $args = array(
            'post_type'      => 'shop_order',
            'post_status'    => 'wc-on-hold',
            'posts_per_page' => -1,
            'meta_query'     => array(
                array(
                    'key'   => '_customer_user',
                    'value' => $user_id,
                ),
            ),
        );

        $orders = get_posts($args);

        // Find the older order with added items to combine with
        $older_order = null;
        foreach ($orders as $order_post) {
            $order_to_combine = wc_get_order($order_post->ID);

            // Check if it's not the same order and it's not already combined
            if ($order_to_combine && $order_to_combine->get_id() !== $order_id && $order_to_combine->get_status() !== 'combined') {
                // Check if shipping addresses match (if present in both orders)
                $current_shipping_address = $current_order->get_address('shipping');
                $combine_shipping_address = $order_to_combine->get_address('shipping');
                if (!empty($current_shipping_address) && !empty($combine_shipping_address) && $current_shipping_address !== $combine_shipping_address) {
                    continue; // Skip combining if shipping addresses don't match
                }

                // Check if delivery dates match (if present in both orders)
                $current_delivery_date = $current_order->get_meta('_orddd_timestamp', true);
                $combine_delivery_date = $order_to_combine->get_meta('_orddd_timestamp', true);
                if (!empty($current_delivery_date) && !empty($combine_delivery_date) && $current_delivery_date !== $combine_delivery_date) {
                    continue; // Skip combining if delivery dates don't match
                }

                // Set the older order with added items to combine with
                $older_order = $order_to_combine;
                break; // Stop after finding the first eligible order
            }
        }

        // If an older order with added items was found, combine orders
        if ($older_order) {
            // Check if the older order is transitioning from 'received' to 'processing'
            if ($older_order->get_status() === 'on-hold' && $current_order->get_status() === 'processing') {
                return;
            }

            // Combine orders by copying items
            $combined_items_list = array();
            foreach ($current_order->get_items() as $item) {
                $product_id = $item->get_product_id();
                $quantity = $item->get_quantity();

                // Check if the product already exists in the combined order
                $existing_item = $older_order->get_item($product_id);

                if ($existing_item) {
                    // If the product exists, update the quantity
                    $existing_quantity = $existing_item->get_quantity();
                    $older_order->set_quantity($existing_item, $existing_quantity + $quantity);
                } else {
                    // If the product does not exist, add it to the combined order
                    $older_order->add_product(wc_get_product($product_id), $quantity);
                }
                
                $combined_items_list[] = $item->get_name() . ' x' . $quantity;
                
                // Restore stock for each item
                wc_update_product_stock($product_id, $quantity, 'increase');
            }
            
            // Copy customer notes with clear delineation
            $custom_order_number = $current_order->get_order_number();
            $customer_note = $current_order->get_customer_note();
            if (!empty($customer_note)) {
                $existing_notes = $older_order->get_customer_note();
                $combined_notes = $existing_notes . "n--- Note from Order #" . $custom_order_number . " ---n" . $customer_note;
                $older_order->set_customer_note($combined_notes);
            }

            // Move the current order with added items to 'combined' status
            $current_order->update_status('combined', __('Orders combined', 'astra'));

            // Add a note to the order log
            $combined_items_log = implode(', ', $combined_items_list);
            $note_combined = sprintf(__('Order combined with Order #%s. Items added: %s', 'astra'), $custom_order_number, $combined_items_log);
            $older_order->add_order_note($note_combined);

            // Log entry for the newer order
            $note_newer = sprintf(__('Items added to Order #%s during combination.', 'astra'), $older_order->get_order_number());
            $current_order->add_order_note($note_newer);
            
            $older_order->calculate_totals();

            // Save changes to the orders
            $older_order->save();
            $current_order->save();
            
            // Fire the update order action hook after the old_order is saved
            wc_reduce_stock_levels($older_order->get_id());
        }
    }
}

// Hook into order status change
add_action('woocommerce_order_status_changed', 'combine_orders_on_status_change', 10, 3);