Woocommerce 产品自定义字段：检查输入是否已存在

我安装了这个“WC Fields Factory”插件来向产品添加新字段，但我需要检查这个新文本框的值是否已存在于数据库中。 举个例子，比如在注册时您不能使用已在使用的电子邮件。

我必须像屏幕上一样添加文本字段

Update 2 - 处理字段验证，仅允许唯一的不存在的文本输入

以下是在单个产品页面中添加自定义文本字段、将其添加为购物车项目数据、在购物车和结账页面中显示、将其另存为订单项目数据以及在订单和电子邮件通知中显示的完整方法：

// HERE define your field label name
function get_field_label_name(){
    return __( "The label name" );
}


// Add a custom product note below product meta in single product pages
add_action('woocommerce_before_add_to_cart_button', 'my_product_custom_field', 100 );
function my_product_custom_field() {

    echo '<div class="my-custom-field">';

    woocommerce_form_field('custom_field1', array(
        'type'        => 'text',
        'class'       => array( 'my-field-class form-row-wide') ,
        'label'       => get_field_label_name(),
        'placeholder' => __("The field placeholder…" ),
        'required'    => true, // Or false
    ) , '');

    echo '</div>';
}

// Check if the custom field value is unique
add_filter( 'woocommerce_add_to_cart_validation', 'wc_add_on_feature', 20, 3 );
function wc_add_on_feature( $passed, $product_id, $quantity ) {
    if( isset($_POST['custom_field1']) && ! empty($_POST['custom_field1']) ){
        global $wpdb;

        $label = get_field_label_name();
        $value = sanitize_text_field( $_POST['custom_field1'] );

        // Check if value exits already
        $result = $wpdb->get_var( "
            SELECT COUNT(meta_value) FROM {$wpdb->prefix}woocommerce_order_itemmeta
            WHERE meta_key LIKE '$label' AND meta_value LIKE '$value'
        " );

        // If it exist we don't allow add to cart
        if( $result > 0 ){
            // Display an error notice
            wc_add_notice( sprintf( __( 'This "%s" input already exist. Please choose another one…' ), $value ), 'error' );
            $passed = false;
        }
    }
    return $passed;
}

// Add custom field value to cart item data
add_filter( 'woocommerce_add_cart_item_data', 'custom_field_value_to_cart_item_data', 20, 2 );
function custom_field_value_to_cart_item_data( $cart_item_data, $product_id ){
    if( isset($_POST['custom_field1']) && ! empty($_POST['custom_field1']) ){
        $cart_item_data['custom_data'] = sanitize_textarea_field( $_POST['custom_field1'] );
    }
    return $cart_item_data;
}


// Display custom cart item data in cart
add_filter( 'woocommerce_get_item_data', 'display_custom_item_data', 10, 2 );
function display_custom_item_data( $cart_item_data, $cart_item ) {

    if ( isset( $cart_item['custom_data'] ) ){
        $cart_item_data[] = array(
            'name' => get_field_label_name(),
            'value' =>  $cart_item['custom_data']
        );
    }
    return $cart_item_data;
}

// Save and display custom field in orders and email notifications (everywhere)
add_action( 'woocommerce_checkout_create_order_line_item', 'custom_field_update_order_item_meta', 20, 4 );
function custom_field_update_order_item_meta( $item, $cart_item_key, $values, $order ) {
    if ( isset( $values['custom_data'] ) ){
        $item->update_meta_data( get_field_label_name(),  $values['custom_data'] );
    }
}

代码位于活动子主题（或活动主题）的 function.php 文件中。经过测试并有效。