stock_extra_fields.php

<?php

// Add Variation Settings
add_action('woocommerce_product_after_variable_attributes', 'variation_settings_fields', 10, 3);

// Save Variation Settings
add_action('woocommerce_save_product_variation', 'save_variation_settings_fields', 10, 2);


// Create new fields for variations
function variation_settings_fields($loop, $variation_data, $variation)
{
    global $wpdb;
    $table_name = "{$wpdb->prefix}wc_warehouse";
    $warehouses = $wpdb->get_results("SELECT code, name FROM $table_name ORDER BY public ASC, sort ASC");
    foreach ($warehouses as $warehouse) {
        $code = stripslashes($warehouse->code);
        $name = stripslashes($warehouse->name);
        $key = $variation->ID . '_'. $code;
        woocommerce_wp_text_input(
      array(
        'id'          => '_warehouse[' . $key .']',
        'label'       => _('Warehouse:') . ' ' . $name,
        'placeholder' => '',
        'description' => '',
        'value'       => get_post_meta(
          $variation->ID,
            'warehouse_' . $code,
            true
        ),
          'custom_attributes' => array(
            'step' 	=> 'any',
            'min'	=> '0'
          )
      )
    );
    }
}



// Save new fields for variations
function save_variation_settings_fields($post_id)
{
    global $wpdb;
    $table_name = "{$wpdb->prefix}wc_warehouse";
    $warehouses = $wpdb->get_results("SELECT code FROM $table_name ORDER BY public ASC, sort ASC");
    $stock = 0;
    foreach ($warehouses as $warehouse) {
        $code = stripslashes($warehouse->code);
        $key = $post_id . '_'. $code;
        $number_field = $_POST['_warehouse'][ $key ];
        if ($number_field === '') {
            $number_field = 0;
        }
        $stock += $number_field;
        update_post_meta(
        $post_id,
          'warehouse_' . $code,
          esc_attr($number_field)
        );
    }
    $variation = new WC_Product_Variation($post_id);
    $variation->set_stock_quantity($stock);
    $variation->save();
}