Query builder question (CI 4 newbie)

Hi there
I started making my first steps with CI4 after years of experience with CI. Now I'm struggling with something that worked fine with query builder prior to CI4.
Here are two functions of a basic model:

PHP Code:

public function getThread($id, $field = NULL) {

 $query = $this->db->table('threads')
              ->where('id', $id)
              ->get();

 $thread = $query->getRow();

 if ($thread == FALSE) {

    return FALSE;
 }

 $thread->posts = $this->basicModel->getPostsByThread($thread->id);

 if (is_null($field)) {

    return $thread;
 }

 else {

    return $thread->$field;
 } 
 }


 public function getPostsByThread($thread_id) {

 $query = $this->db->table('posts')
              ->where('thread_id', $thread_id)
              ->orderBy('date')
              ->get();

 return $query->getResult();
 } 


So, when calling function getThread() CI4 return the following error caused by the line $thread->posts = …:

CodeIgniter\Database\Exceptions\DatabaseException #8
You must set the database table to be used with your query.

I guess I have to rethink using query builder. Maybe you can give me a hint.