【WordPress】プラグインなしで関連記事を表示する方法｜transientキャッシュでorderby=randの負荷を軽減

$current_post_id = get_the_ID();
$categories       = wp_get_post_categories( $current_post_id );

if ( $categories ) {
    $args = array(
        'category__in'   => $categories,
        'post__not_in'   => array( $current_post_id ),
        'posts_per_page' => 5,
        'orderby'        => 'rand',
        'no_found_rows'  => true, // ページネーション用の件数計算を省略
    );

    $related_posts = new WP_Query( $args );

    if ( $related_posts->have_posts() ) {
        echo '<div class="related-posts">';
        echo '<h3>関連記事</h3><ul>';

        while ( $related_posts->have_posts() ) {
            $related_posts->the_post();
            ?>
            <li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
            <?php
        }

        echo '</ul></div>';
        wp_reset_postdata();
    }
}

$current_post_id = get_the_ID();
$cache_key        = 'related_posts_' . $current_post_id;
$related_ids      = get_transient( $cache_key );

if ( false === $related_ids ) {
    $categories = wp_get_post_categories( $current_post_id );
    $related_ids = array();

    if ( $categories ) {
        $query = new WP_Query( array(
            'category__in'   => $categories,
            'post__not_in'   => array( $current_post_id ),
            'posts_per_page' => 5,
            'orderby'        => 'rand',
            'no_found_rows'  => true,
            'fields'         => 'ids', // IDだけ取得して軽量化
        ) );

        $related_ids = $query->posts;
    }

    set_transient( $cache_key, $related_ids, DAY_IN_SECONDS ); // 1日キャッシュ
}

if ( $related_ids ) {
    echo '<div class="related-posts"><h3>関連記事</h3><ul>';
    foreach ( $related_ids as $post_id ) {
        echo '<li><a href="' . esc_url( get_permalink( $post_id ) ) . '">'
            . esc_html( get_the_title( $post_id ) ) . '</a></li>';
    }
    echo '</ul></div>';
}

$categories = wp_get_post_categories( get_the_ID() );

if ( $categories ) {
    $query = new WP_Query( array(
        'category__in'   => $categories,
        'post__not_in'   => array( get_the_ID() ),
        'posts_per_page' => 20, // 候補を多めに取得
        'orderby'        => 'date', // インデックスが効く通常の並び順
        'no_found_rows'  => true,
        'fields'         => 'ids',
    ) );

    $ids = $query->posts;
    shuffle( $ids ); // PHP側でランダム化
    $related_ids = array_slice( $ids, 0, 5 );
}

.related-posts {
  margin-top: 20px;
}
.related-posts h3 {
  font-size: 1.5em;
  margin-bottom: 10px;
}
.related-posts ul {
  list-style-type: none;
  padding: 0;
}
.related-posts ul li {
  margin-bottom: 5px;
}
.related-posts ul li a {
  text-decoration: none;
  color: #0073aa;
}
.related-posts ul li a:hover {
  text-decoration: underline;
}