Shortcode to return values from a 1:1 ACF relationship

acf_relationship_1to1_shortcode.php

<?php

function acf_relationship_shortcode($atts) {
    $atts = shortcode_atts(array(
        'name' => '',        // ACF field name for the relationship
        'display' => '',     // ACF field name or WordPress field to display from the related post
        'size' => 'full'     // OPTIONAL - Size of the featured image (defaults to 'full')
    ), $atts);

    if (empty($atts['name']) || empty($atts['display'])) {
        return 'Missing attributes for the shortcode.';
    }

    $related_posts = get_field($atts['name']);

    if (!empty($related_posts)) {
        // Since it's a 1:1 relationship, we'll get the first related post
        $related_post = $related_posts[0];

        // Handling basic WordPress default fields
        switch ($atts['display']) {
            case 'title':
                $field_value = get_the_title($related_post->ID);
                break;
            case 'url':
                $field_value = get_permalink($related_post->ID);
                break;
            case 'content':
                $field_value = apply_filters('the_content', get_post_field('post_content', $related_post->ID));
                break;
            case 'featured_image':
                $field_value = get_the_post_thumbnail_url($related_post->ID, $atts['size']);
                break;
            default:
                // If not a default WP field, get ACF field value from the related post
                $field_value = get_field($atts['display'], $related_post->ID);
                break;
        }

        return $field_value;
    }

    return 'No related posts found.';
}

// Register the shortcode
add_shortcode('acf_relationship', 'acf_relationship_shortcode');

// Shortcode format
// [acf_relationship name="your_relationship_field_name" display="featured_image" size="thumbnail"]