Wordpress - setting a custom post per category - wordpress

Question

I have added the below code to the functions.php file to load different post templates per category. The code is doing what it is supposed to do. The issue I am facing is that when I open posts these load correctly but whilst loading an error is displayed for a instant saying "trying to get property of non-object in .......functions.php" How may I get rid of that error? Appreciate your help.
/* ----------------------------------------------------------------------------
custom single posts per category
*/
add_filter('single_template', 'check_for_category_single_template');
function check_for_category_single_template( $t )
{
foreach( (array) get_the_category() as $cat )
{
if ( file_exists(get_stylesheet_directory() . "/single-category-{$cat->slug}.php") ) return get_stylesheet_directory() . "/single-category-{$cat->slug}.php";
if($cat->parent)
{
$cat = get_the_category_by_ID( $cat->parent );
if ( file_exists(get_stylesheet_directory() . "/single-category-{$cat->slug}.php") ) return get_stylesheet_directory() . "/single-category-{$cat->slug}.php";
}
}
return $t;
}

Answer 1

The error trying to get property of non-object means, you try to get a property of something that is not an object. So looking at your code you are getting two properties with ->: "slug" and "parent".
This gives us the hint, that the value saved in the $cat variable might not be an object.
You are using get_the_category_by_ID which gives you a name, not a category object. https://developer.wordpress.org/reference/functions/get_the_category_by_id/ So you cannot access any property. The problem is this: get_the_category_by_ID( $cat->parent ); There is no parent property in $cat, because it only contains the name. You need to get the object to make use of the parent property. Moreover the property is called category_parent, and not parent.
$parent_id = $cat->category_parent;
$cat = get_category($parent_id);
This should solve your problem.
If it does not solve your problem, I wrote the code in another way. It shows single steps and maybe it is useful for you or some other users.
The function to get all your category objects is: get_categories() https://developer.wordpress.org/reference/functions/get_categories/
You can save all the IDs of the categoires into a array. After that, you loop through each position of the array and check if the single post you are viewing has the same category ID. We can check if the post is in given categories using in_category() https://developer.wordpress.org/reference/functions/in_category/
I did no testing, but the code can look something like:
add_filter('single_template', 'check_for_category_single_template');
function check_for_category_single_template() {
// Get all category objects
$categories = get_categories();
// Array for IDs
$categories_ids = [];
foreach($categories as $category) {
// add id to array
array_push($categories_ids, $category->cat_ID);
}
// Loop through array of IDs
foreach($categories_ids as $category_id) {
// Check if ID equals current post category ID
if ( in_category($category_id) ) {
// get current category object
$cat = get_category($category_id);
// check if category has parent
if( $cat->category_parent > 0 ) {
$parent_id = $cat->category_parent;
// get parent object
$cat = get_category( $parent_id );
}
// get slug of category
$cat_slug = $cat->slug;
if ( file_exists(get_stylesheet_directory() . "/single-category-" .$cat_slug. ".php") ) {
return get_stylesheet_directory() . "/single-category-" .$cat_slug. ".php";
}
}
}
}