ACF Reciprocal Relationship Functionality

gistfile1.aw

<?php
function update_value( $post_id, $field, $value ){
		
	// Check to see if there are any old relationships
	if( parent::get_value( $post_id, $field ) !== '' ){
		
		// If so, find out what needs to be added or deleted
		$old_value = explode( ',', parent::get_value( $post_id, $field ) );
				
		$new_value = explode( ',', $value );
				
		// Determine which posts we need to remove the reciprocal relationships to
		$posts_to_remove_relationships_from = array_diff( $old_value, $new_value );
			
		// Determine which posts we need to add the reciprocal relationships to
		$posts_to_add_relationships_to = array_diff( $new_value, $old_value );
			
	} else {
						
		// There are no old relationships, so the are all new
		$posts_to_remove_relationships_from = array();
				
		$posts_to_add_relationships_to = explode( ',', $value );
				
	}
			
	// Add relationships
	foreach( $posts_to_add_relationships_to as $post_to_add_relationship_to ){
			
		// Get the current value of the related post
		$related_post_relationships = explode( ',', get_post_meta( $post_to_add_relationship_to, $field['name'], true ) );
				
		if( $related_post_relationships[0] === '' ){
										
			// This is post is new to relationships. Just add this post id
			$related_post_relationships = array( $post_id );
					
		} else {
					
			// If the current post id isn't already related, append it to the list
			if( ! in_array( $post_id, $related_post_relationships ) ) {
						
				$related_post_relationships[] = $post_id;

			}
					
		}
				
		// Create a string out of the results
		$related_post_relationships = implode( ',', $related_post_relationships );
								
		// Update the related post
		update_post_meta( $post_to_add_relationship_to, $field['name'], $related_post_relationships );
		
				
	}
			
	// Remove relationships
	foreach( $posts_to_remove_relationships_from  as $post_to_remove_relationship_from ){
			
		// Get the current value of the related post
		$related_post_relationships = explode( ',', get_post_meta( $post_to_remove_relationship_from, $field['name'], true ) );
				
		// Loop through the array, and unset the post id
		foreach( $related_post_relationships as $key => $related_post_relationship ){
				
			if( (int) $related_post_relationship === (int) $post_id ){
						
				unset( $related_post_relationships[$key] );
						
			}
					
		}
				
		$related_post_relationships = implode( ',', $related_post_relationships );
				
		// Update the related post
		update_post_meta( $post_to_remove_relationship_from, $field['name'], $related_post_relationships );
			
	}
						
	// Save Value
	parent::update_value( $post_id, $field, $value );
			
}
?>