Jetpack: Add Related Posts to your RSS feed

I got an interesting question in the Jetpack support forums today. Levy wanted to display Related Posts in their RSS feed.

Jetpack displays Related Posts at the bottom of single posts by default, but like with other modules, you can customize Related Posts. In this post, we’ll use the the_content filter and the raw Jetpack Related Posts class to build our own unordered list of Related Posts, and add it to the bottom of the post content in RSS feeds.

Quick note before we start:

• You’ll want to add that code to a functionality plugin like this one.
• Nothing will happen if you’re not using Jetpack and its Related Posts module.

/**
 * Add Jetpack Related Posts to RSS feed.
 *
 * @see https://wordpress.org/support/topic/2927523
 *
 * @param string $content Post content.
 */
function jeherve_related_posts_feed( $content ) {
    // Return early if we're not in the RSS feed.
    if ( ! is_feed() ) {
        return $content;
    }

    // If Jetpack and Related Posts are active, let's get started.
    if ( class_exists( 'Jetpack_RelatedPosts' ) && method_exists( 'Jetpack_RelatedPosts', 'init_raw' ) ) {
        $related = Jetpack_RelatedPosts::init_raw()
            ->set_query_name( 'jetpackme-shortcode' ) // Optional, name can be anything.
            ->get_for_post_id(
                get_the_ID(),
                array( 'size' => 3 )
            );

        if ( $related ) {
            $related_list = '';

            foreach ( $related as $result ) {
                // Get the related post IDs.
                $related_post_id = get_post( $result['id'] );

                /**
                 * From there you can do just about anything, using the post IDs.
                 *
                 * In this example, we'll build an unordered list.
                 */
                $related_list .= sprintf(
                    '<li><a title="%1$s" href="%2$s">%3$s</a></li>',
                    esc_attr( get_the_title( $related_post_id ) ),
                    get_permalink( $related_post_id ),
                    get_the_title( $related_post_id )
                );
            }

            /**
             * Let's wrap all those related posts in ul tags, and add that list to the end of our post content.
             *
             * We will also add a headline, but only if it was set to be displayed in your Jetpack Related Posts settings.
             */
            $related_options = Jetpack_Options::get_option( 'relatedposts' );
            if ( $related_options['show_headline'] ) {
                $headline = sprintf(
                    '<h3 class="jp-relatedposts-headline"><em>%s</em></h3>',
                    esc_html__( 'Related', 'jetpack' )
                );
            } else {
                $headline = '';
            }

            return sprintf(
                '%1$s%2$s<ul class="jp-relatedposts">%3$s</ul>',
                $content,
                apply_filters( 'jetpack_relatedposts_filter_headline', $headline ),
                $related_list
            );
        }

        return $content;
    }

    // Last fallback, just in case Jetpack and Related Posts aren't there anymore.
    return $content;
}
add_filter( 'the_content', 'jeherve_related_posts_feed' );