Nepravidelný PDOException No reference found for…

Dobry vecer.

Uz dobre 2 tyzdne ma trapi neskutocna chyba, s ktorou si neviem vobec dat rady a uz som z toho zufaly. Jednoducho v nepravidelnych intervaloch, neviem z akeho dovodu sa dostavam k chybe „No reference found for…“.

Dam konkretny priklad, kde sa tato chyba vyskytuje. Cize je to No reference found for $courses->icon_interactive. Cize premennu $courses vytvaram zo selectu

$this->db->table('courses')->where(array('id'=>$id))->fetch();

, v ktorom si vyberam z jednej tabulky udaje o danom kurze. Lenze k tymto udajom, cize k tomuto polu $courses potrebujem dostat udaje z dalsich 3 tabuliek (ceny,ikonky,kategorie). Robim to nasledovnym sposobom:

//vyberiem si udaje o ikonkach z DB
$icons = $this->db->table('courses_icons')->where('courses_id',$id)->fetch();

//a priradim ich do poli v $courses
		$course->icon_interactive = $icons->icon_interactive;
		$course->icon_quiz = $icons->icon_quiz;
		$course->icon_music = $icons->icon_music;
		$course->icon_video = $icons->icon_video;
		$course->icon_files = $icons->icon_files;
		$course->icon_tutor = $icons->icon_tutor;
		$course->icon_case_study = $icons->icon_case_study;
		$course->icon_certificate = $icons->icon_certificate;

$categories = $this->db->table('courses_in_categories')->where('courses_id',$id);

foreach($categories as $category){
				$cat[] = $category['category_id'];
			}
			$course->categories = $cat;

//pod. sposobom este ceny......... nepodstatne

Tymto sposobom potrebujem pouzivat v sablonach $courses->icon_interactive, $course->icon_tutor, ......
Kde je chyba? V com moze byt problem?

Dakujem pekne za rychle a funkcne riesenie