Module development and code questions

I am porting a D7 module that i want to release for D8. 

The module injects a block that you can select into the body. The problem i have is that i achived to manipulate the body field and save it, BUT it is really saved to the DB not only for the display. That is very bad as i only want the injection to be happen at display withoput changing the original body content.

 protected function buildQuery($ids, $conditions = array(), $revision_id = FALSE) {

    $query = parent::buildQuery($ids, $conditions, $revision_id);

    if ($this->pessimisticLocking && isset($this->entityInfo['locking mode']) && $this->entityInfo['locking mode'] == 'pessimistic') {

      // In pessimistic locking mode, we issue the load query with a FOR UPDATE

      // clause. This will block all other load queries to the loaded objects

      // but requires us to start a transaction.

Hey there, 

I did a quick search and couldn't find much about upgrading/updating modules to Drupal 9. I'm sorry if I missed it, please just point me to right direction. 

Long time Drupal user (5.x was my first experiment with Drupal and I've been on Drupal since the early days of 6 - not using Drush - yet ).

I need some guidance about Drupal 8.x modules/themes upgrading to Drupal 9.

I'm ready to move on to Drupal 9 (currently 8.9) but my theme and slider are not compatible (or at least, have not been updated for 9). Suggestions?

I am trying to retrieve an object's protected key/value

I'm trying to get a specific menu link to a node but we have a giant menu structure with more than one link to the same node.

$menu_links = $menu_link_service->loadLinksByRoute('entity.node.canonical', $route_params, $menu_name);

This code works but $menu_links sometimes has multiple links.  The Menu UI module somehow keeps track of only a single menu link even if there are multiple links in the same menu to that node. I'm calling this the menu ui link, though maybe there's a better way to refer to it.

Hello Everyone, 

when I'm calling system_retrieve_file like:

$thumbnail = system_retrieve_file($thumbnail_url, NULL, TRUE, FileSystemInterface::EXISTS_REPLACE);

It will download the image into file system but when I try to print $thumbnail object it will have empty fid (file id). 

Note: when I ran the same code again with the same thumbnail url, it gave me the fid. 

Not sure why at first place fid is empty.