Modified version of the bidirectional relationship function to help facilitate users to cpt relationships

acf-sync-user-and-cpt-bidirectional-relationship.php

<?php

/*
 * This code is modified from the original here 
 * https://github.com/Hube2/acf-filters-and-functions/blob/master/acf-reciprocal-relationship.php
 *
 * Some comments may not be useful anymore since the modifications
 */


function dod_dale_sync_users_and_groups($value, $post_id, $field) {
	
	// set the two fields that you want to create
	// a two way relationship for
	// these values can be the same field key
	// if you are using a single relationship field
	// on a single post type
	
	// the field key of one side of the relationship - CPT
	$key_a = 'field_5a04f302ce2ae';

	// the field key of the other side of the relationship - USER
	$key_b = 'field_5a61019589d9b';

	//set the type so we handle the differences in users and cpts ids
	$type = 'post';
	
	// figure out wich side we're doing and set up variables
	// if the keys are the same above then this won't matter
	// $key_a represents the field for the current posts
	// and $key_b represents the field on related posts
	if ($key_a != $field['key']) {
		// this is side b, swap the value
		$temp = $key_a;
		$key_a = $key_b;
		$key_b = $temp;

		$type = 'user';
		//when the field is a user we need to remove user_ from post_id
		$post_id = substr($post_id, strpos($post_id, '_') + 1);
	}
	
	// get both fields
	// this gets them by using an acf function
	// that can gets field objects based on field keys
	// we may be getting the same field, but we don't care
	$field_a = acf_get_field($key_a);
	$field_b = acf_get_field($key_b);
	
	// set the field names to check
	// for each post
	$name_a = $field_a['name'];
	$name_b = $field_b['name'];
	
	// get the old value from the current post
	// compare it to the new value to see
	// if anything needs to be updated
	// use get_post_meta() to a avoid conflicts
	if ('user' == $type) {
		$old_values = get_user_meta($post_id, $name_a, true);
	} else {
		$old_values = get_post_meta($post_id, $name_a, true);
	}
	
	// make sure that the value is an array
	if (!is_array($old_values)) {
		if (empty($old_values)) {
			$old_values = array();
		} else {
			$old_values = array($old_values);
		}
	}
	// set new values to $value
	// we don't want to mess with $value
	$new_values = $value;
	// make sure that the value is an array
	if (!is_array($new_values)) {
		if (empty($new_values)) {
			$new_values = array();
		} else {
			$new_values = array($new_values);
		}
	}
	
	// get differences
	// array_diff returns an array of values from the first
	// array that are not in the second array
	// this gives us lists that need to be added
	// or removed depending on which order we give
	// the arrays in
	
	// this line is commented out, this line should be used when setting
	// up this filter on a new site. getting values and updating values
	// on every relationship will cause a performance issue you should
	// only use the second line "$add = $new_values" when adding this
	// filter to an existing site and then you should switch to the
	// first line as soon as you get everything updated
	// in either case if you have too many existing relationships
	// checking end updated every one of them will more then likely
	// cause your updates to time out.
	//$add = array_diff($new_values, $old_values);
	$add = $new_values;
	$delete = array_diff($old_values, $new_values);
	
	// reorder the arrays to prevent possible invalid index errors
	$add = array_values($add);
	$delete = array_values($delete);
	
	if (!count($add) && !count($delete)) {
		// there are no changes
		// so there's nothing to do
		return $value;
	}
	
	// do deletes first
	// loop through all of the posts that need to have
	// the recipricol relationship removed
	for ($i=0; $i<count($delete); $i++) {
		if ('post' == $type) {
			$related_values = get_user_meta($delete[$i], $name_b, true);
		} else {
			$related_values = get_post_meta($delete[$i], $name_b, true);
		}
		if (!is_array($related_values)) {
			if (empty($related_values)) {
				$related_values = array();
			} else {
				$related_values = array($related_values);
			}
		}
		// we use array_diff again
		// this will remove the value without needing to loop
		// through the array and find it
		$related_values = array_diff($related_values, array($post_id));

		if ('post' == $type) {
			// insert the new value
			update_user_meta($delete[$i], $name_b, $related_values);
			// insert the acf key reference, just in case
			update_user_meta($delete[$i], '_'.$name_b, $key_b);
		} else {
			// insert the new value
			update_post_meta($delete[$i], $name_b, $related_values);
			// insert the acf key reference, just in case
			update_post_meta($delete[$i], '_'.$name_b, $key_b);
		}
	}
	
	// do additions, to add $post_id
	for ($i=0; $i<count($add); $i++) {
		if ('post' == $type) {
			$related_values = get_user_meta($add[$i], $name_b, true);
		} else {
			$related_values = get_post_meta($add[$i], $name_b, true);
		}
		if (!is_array($related_values)) {
			if (empty($related_values)) {
				$related_values = array();
			} else {
				$related_values = array($related_values);
			}
		}
		if (!in_array($post_id, $related_values)) {
			// add new relationship if it does not exist
			$related_values[] = $post_id;
		}

		if ('post' == $type) {
			// insert the new value
			update_user_meta($add[$i], $name_b, $related_values);
			// insert the acf key reference, just in case
			update_user_meta($add[$i], '_'.$name_b, $key_b);
		} else {
			// insert the new value
			update_post_meta($add[$i], $name_b, $related_values);
			// insert the acf key reference, just in case
			update_post_meta($add[$i], '_'.$name_b, $key_b);
		}
	}
	
	return $value;
	
}
add_filter('acf/update_value/key=field_5a04f302ce2ae', 'dod_dale_sync_users_and_groups', 10, 3);
add_filter('acf/update_value/key=field_5a61019589d9b', 'dod_dale_sync_users_and_groups', 10, 3);