ACF Bidirectional Relationschip

// Pas alleen '=bidirectional_relationship' aan naar de juiste veldnaam. werkt ook tussen verschillende CPT's. veld kan apart aangemaakt worden in verschillende groepen, als het maar dezelfde naam heeft.

function bidirectional_acf_update_value( $value, $post_id, $field  ) {
  
  // vars
  $field_name = $field['name'];
  $global_name = 'is_updating_' . $field_name;
  
  
  // bail early if this filter was triggered from the update_field() function called within the loop below
  // - this prevents an inifinte loop
  if( !empty($GLOBALS[ $global_name ]) ) return $value;
  
  
  // set global variable to avoid inifite loop
  // - could also remove_filter() then add_filter() again, but this is simpler
  $GLOBALS[ $global_name ] = 1;
  
  
  // loop over selected posts and add this $post_id
  if( is_array($value) ) {
  
    foreach( $value as $post_id2 ) {
      
      // load existing related posts
      $value2 = get_field($field_name, $post_id2, false);
      
      
      // allow for selected posts to not contain a value
      if( empty($value2) ) {
        
        $value2 = array();
        
      }
      
      
      // bail early if the current $post_id is already found in selected post's $value2
      if( in_array($post_id, $value2) ) continue;
      
      
      // append the current $post_id to the selected post's 'related_posts' value
      $value2[] = $post_id;
      
      
      // update the selected post's value
      update_field($field_name, $value2, $post_id2);
      
    }
  
  }
  
  
  // find posts which have been removed
  $old_value = get_field($field_name, $post_id, false);
  
  if( is_array($old_value) ) {
    
    foreach( $old_value as $post_id2 ) {
      
      // bail early if this value has not been removed
      if( is_array($value) && in_array($post_id2, $value) ) continue;
      
      
      // load existing related posts
      $value2 = get_field($field_name, $post_id2, false);
      
      
      // bail early if no value
      if( empty($value2) ) continue;
      
      
      // find the position of $post_id within $value2 so we can remove it
      $pos = array_search($post_id, $value2);
      
      
      // remove
      unset( $value2[ $pos] );
      
      
      // update the un-selected post's value
      update_field($field_name, $value2, $post_id2);
      
    }
    
  }
  
  
  // reset global varibale to allow this filter to function as per normal
  $GLOBALS[ $global_name ] = 0;
  
  
  // return
    return $value;
    
}

add_filter('acf/update_value/name=bidirectional_relationship', 'bidirectional_acf_update_value', 10, 3);