Wordpress: Custom Post Type with Required, Unique Custom Field Sample Plugin

custom_post_type_with_required_custom_field.php

<?php

/*
Plugin Name: Custom Post Type With Required Custom Field
Plugin URI: http://mac-blog.org.ua/
Description: Sample plugin demonstrating how to add required unique custom field to custom post type
Author: Marchenko Alexandr
Version: 1.0
Author URI: http://mac-blog.org.ua/
*/

define('CPTWRCF_POST_TYPE', 'video');
define('CPTWRCF_META_KEY', '_video');
define('CPTWRCF_META_LABEL', 'Video URL');

define('CPTWRCF_NONCE_ACTION', 'cptwrcf_nonce_action');
define('CPTWRCF_NONCE_NAME', 'cptwrcfn_nonce_name');

define('CPTWRCF_NOT_VALID_QUERY_STRING_KEY', 'cptwrcfn_not_valid');
define('CPTWRCF_NOT_UNIQUE_QUERY_STRING_KEY', 'cptwrcfn_not_unique');

define('CPTWRCF_SQL', "SELECT * FROM $wpdb->postmeta WHERE meta_key = %s AND meta_value = %s AND post_id <> %d");

// First things first we are going to create custom post type
add_action('init', function() {
    register_post_type(CPTWRCF_POST_TYPE, [
        'label' => 'Video',
        'public' => true,
        'supports' => ['title', 'editor', 'thumbnail', 'excerpt', 'comments'],
        'show_in_menu' => 'edit.php',
        'menu_icon' => 'dashicons-video-alt3',
        //'taxonomies' => [''],
        'has_archive' => true,
    ]);
});

// And add custom meta box for its Video URL custom field
add_action('add_meta_boxes', function() {
    add_meta_box(CPTWRCF_META_KEY, CPTWRCF_META_LABEL, function(WP_Post $post) {
        wp_nonce_field(CPTWRCF_NONCE_ACTION, CPTWRCF_NONCE_NAME);

        $attributes = [
            'id' => CPTWRCF_META_KEY,
            'name' => CPTWRCF_META_KEY,
            'value' => get_post_meta($post->ID, CPTWRCF_META_KEY, true),
            'class' => 'widefat',
            'type' => 'text',
            //'type' => 'url',
            //'required' => 'required'
        ];

        $attributes = implode(' ', array_map(function($key) use($attributes) {
            return sprintf('%s="%s"', $key, esc_attr($attributes[$key]));
        }, array_keys($attributes)));

        echo '<input ' . $attributes . ' />';
    }, CPTWRCF_POST_TYPE, 'side', 'core');
});

// Handle post save event and save Video URL custom field as post meta
add_action('save_post', function($post_id) {
    if (!isset($_POST[CPTWRCF_NONCE_NAME])) return;
    if (!wp_verify_nonce($_POST[CPTWRCF_NONCE_NAME], CPTWRCF_NONCE_ACTION)) return;
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return;
    if (!isset($_POST[CPTWRCF_META_KEY])) return;
    if (!current_user_can('edit_post', $post_id)) return;

    update_post_meta($post_id, CPTWRCF_META_KEY, sanitize_text_field($_POST[CPTWRCF_META_KEY]));
});


// All that was pretty standard stuff with nothing special
// Now we are going to add two additional capabilities:
// 1. Make Video URL custom field - required
// 2. Ensure that it is unique across all posts

// REQUIRED

// <editor-fold desc="REQUIRED">
add_filter('wp_insert_post_data', function (array $data) {
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return $data;
    if(!empty($_GET['action']) && $_GET['action'] === 'trash') return $data; // Make sure to do nothing for posts that are going to be deleted

    // If there is not Video URL or it is not valid URL - mark post as draft and notify user
    if(empty($_POST[CPTWRCF_META_KEY]) || !filter_var($_POST[CPTWRCF_META_KEY], FILTER_VALIDATE_URL)) {
        $data['post_status'] = 'draft';
        add_filter('redirect_post_location', function($location) {
            $location = remove_query_arg('message', $location);
            $location = add_query_arg('message', 10, $location); // 10 is for "Post draft updated" message
            return add_query_arg(CPTWRCF_NOT_VALID_QUERY_STRING_KEY, 1, $location);
        });
    } else {
        add_filter('redirect_post_location', function($location) {
            return remove_query_arg(CPTWRCF_NOT_VALID_QUERY_STRING_KEY, $location);
        });
    }

    return $data;
});

add_action('admin_notices', function () {
    if (isset($_GET[CPTWRCF_NOT_VALID_QUERY_STRING_KEY])) {
        $link = sprintf('<b><a href="#%s">%s</a></b>', CPTWRCF_META_KEY, CPTWRCF_META_LABEL);
        $message = sprintf(__('Your post was saved as draft because there is no required %s or it is invalid!'), $link);
        echo sprintf('<div class="error"><p>%s</p></div>', $message);
    }
});
// </editor-fold>

// UNIQUE

// <editor-fold desc="UNIQUE">
add_filter('wp_insert_post_data', function (array $data, array $raw) {
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return $data;
    if(!empty($_GET['action']) && $_GET['action'] === 'trash') return $data; // Make sure to do nothing for posts that are going to be deleted

    $post_id = (empty($data['ID']) ? $raw['ID'] : $data['ID']) ?: 0;

    /** @var wpdb $wpdb */
    global $wpdb;
    $query = $wpdb->prepare(CPTWRCF_SQL, CPTWRCF_META_KEY, @$_POST[CPTWRCF_META_KEY], $post_id);
    $found = $wpdb->get_results($query, ARRAY_A);

    // If we found posts with save Video URL - notify user and save post as a draft
    if($found) {
        $data['post_status'] = 'draft';
        add_filter('redirect_post_location', function ($location){
            $location = remove_query_arg('message', $location);
            $location = add_query_arg('message', 10, $location); // 10 is for "Post draft updated" message from `edit-form-advanced.php`
            return add_query_arg(CPTWRCF_NOT_UNIQUE_QUERY_STRING_KEY, 1, $location);
        });
    } else {
        add_filter('redirect_post_location', function ($location){
            return remove_query_arg(CPTWRCF_NOT_UNIQUE_QUERY_STRING_KEY, $location);
        });
    }

    return $data;
}, 10, 2); // Notice last arg: 2 - is for getting raw data which is used to determine post id

add_action('admin_notices', function () {
    if (isset($_GET[CPTWRCF_NOT_UNIQUE_QUERY_STRING_KEY])) {
        /** @var wpdb $wpdb */
        global $wpdb;

        /** @var WP_Post $post */
        global $post;

        // <editor-fold desc="Optional: Get post links that has same Video URL">
        $meta_value = get_post_meta($post->ID, CPTWRCF_META_KEY, true);
        $query = $wpdb->prepare(CPTWRCF_SQL, CPTWRCF_META_KEY, $meta_value, $post->ID);
        $items = $wpdb->get_results($query, ARRAY_A);
        $items = array_map(function ($item) {
            return sprintf('<a target="_blank" href="%s">%s</a>', get_permalink($item['post_id']), get_the_title($item['post_id']));
        }, $items);
        $items = implode(', ', $items);
        $used_by = sprintf('by %s', $items);
        // </editor-fold>

        $link = sprintf('<b><a href="#%s">%s</a></b>', CPTWRCF_META_KEY, CPTWRCF_META_LABEL);
        $message = sprintf(__('Your post was saved as draft because %s is already used %s'), $link, $used_by);
        echo sprintf('<div class="error"><p>%s</p></div>', $message);
    }
});
// </editor-fold>