WordPress | ACF | Selected or fallback

selected_or_fallback.php

<?php
/**
* Works with the ACF relationship field to always display the quota of posts
*/
function selected_or_fallback( $current_id, $total_required, $acf_relationship_name, $post_type = null) {
    $excluded = array( $current_id );
    $obtained = 0;
    $related = array();

    // Check to see if any posts have been selected via ACF relationship
    $selected = get_field( $acf_relationship_name );
    if( $selected ) {
        $related = $selected;
        $obtained = count( $selected );
        if ( $obtained < $total_required ) {
            // If the quota not met update excluded array for get_posts query
            $selected_ids = get_field($acf_relationship_name, false, false); // Returns an array of IDs
            $excluded = array_merge($selected_ids, $excluded);
        }
    }

    if ( $obtained < $total_required ) {
        if( $selected ) {
            $post_type = get_post_type( $selected_ids[0] );
        } else {
            if ( !$post_type ) {
                $post_type = get_post_type( $current_id );
            }
        }
        $required = $total_required - $obtained;
        $args = array(
            'posts_per_page' => $required,
            'post_type' => $post_type,
            'exclude' => $excluded
        );
        $posts = get_posts( $args );
        if( $posts ) {
            $related = array_merge($related, $posts);
        }
    }
    return $related;
}

Update May 2020

I noticed that if this was used on a page or a post type that didn't match the intended display of featured items, it would grab additional items based on the post-type of the current post/page. So, for example, if this was used on the homepage of the site and only 1 post was selected, the other two would be pages.

I've therefore adapted the gist so that it takes the post type from the first item selected as opposed to the post-type of the current id.

Update October 2020

Further update allowing you to pass in a post-type variable. Whilst the last update was good, it failed where no posts were selected on somewhere like the homepage as the function wouldn't be able to determine the intended post-type and would instead display pages.