ACF Relational Field Shortcode Expanded

functions.php

/**
 * Output an ACF post relationship field type using a shortcode:
 * [acf_relationship_field field="field_name" show="all"]
 * You can also pass `post_id` as an attribute
 * 
 * Show parameter accepts any comma seperated combination of: all, title, title_linked, date, author, post_excerpt, post_content
 *
 * Works with Post Object and Relationship fields, when the return
 * format is both post object and post ID
 *
 * This code assumes you are copying this into functions.php of
 * your child theme.
**/

// Setup the shortcode
add_shortcode( 'acf_relationship_field', 'shortcode_get_relationship_field' ); 

function shortcode_get_relationship_field( $atts ) {
	ob_start();
	
	global $post;
	$defaults = array(
		'post_id' => $post->ID, // Default to the current Post ID.
		'field'   => '',
		'show'    => 'all', // Default to showing all elements
	);
	
	$args = wp_parse_args( $atts, $defaults );
	
	$related_posts = get_field( $args['field'], $args['post_id'] );

	// Check if we have a single post (post object, post ID) or an
	// array of posts then convert to array.
	if( is_object( $related_posts ) ) {
		$related_posts = array( $related_posts );
	} else if ( is_scalar( $related_posts ) ) {
		$related_posts = array( $related_posts );
	}
	if ( $related_posts ) {
		foreach( $related_posts as $related_post ) {
			if ( is_scalar( $related_post ) ) {
				$related_post = get_post( $related_post );
			}
			?>
		    <div class="related-post">
		    	<?php 
		    		if (strpos($args['show'], 'title') !== false) {
		    			if (strpos($args['show'], 'linked') !== false) {
		    				echo '<h2 class="related-post-title"><a href="' . esc_url( get_permalink( $related_post->ID ) ) . '">' . esc_html( get_the_title( $related_post->ID ) ) . '</a></h2>';
		    			} else {
		    				echo '<h2 class="related-post-title-unlinked">' . esc_html( get_the_title( $related_post->ID ) ) . '</h2>';
		    			}
		    		}
		    		if (strpos($args['show'], 'date') !== false) {
		    			echo '<p class="related-post-date">' . esc_html( get_the_date( '', $related_post->ID ) ) . '</p>';
		    		}
		    		if (strpos($args['show'], 'author') !== false) {
		    			echo '<p class="related-post-author">' . esc_html( get_the_author_meta( 'display_name', $related_post->post_author ) ) . '</p>';
		    		}
		    		if (strpos($args['show'], 'post_excerpt') !== false) {
		    			echo '<p class="related-post-excerpt">' . esc_html( get_the_excerpt( $related_post->ID ) ) . '</p>';
		    		}
		    		if (strpos($args['show'], 'post_content') !== false) {
		    			echo '<div class="related-post-content">' . apply_filters( 'the_content', $related_post->post_content ) . '</div>';
		    		}
		    	?>
		    </div>
		<?php } ?>
	<?php
	}
	return ob_get_clean();
}

Thanks to this gist for getting me started.

Example usage:
[acf_relationship_field field="your_field_name" show="title_linked,date,author,post_excerpt"]

Accepted show parameters:
all -> shows post title (linked), post date, post author, post excerpt
title -> shows the post title not linked
title_linked -> shows the post title linked to the post
date -> shows the post date
author -> shows the post author (not linked)
post_excerpt -> shows the post excerpt
post_content -> shows the full post content