cant set stored property with associated object in extention

I have custom class, and I want to extend it and add stored property, one of solutions I found is to do it with Associated objects. My Code looks like that:

import ObjectiveC

var kSomeKey = "s"

extension ProductCategory {
    var parent: ProductCategory? {
        get {
            return objc_getAssociatedObject(self, &kSomeKey) as? ProductCategory
        }
        set(newValue) {
            objc_setAssociatedObject(self, &kSomeKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            print(parent ?? "not set")
        }
    }
}

I make setting of this property like that:

 private func makeConnectionsWithParents(forArray sorce: ProductCategory) {
    for var cheesItem in sorce.childCategories! {
        if cheesItem.childCategories != nil {
           cheesItem.parent = sorce
            makeConnectionsWithParents(forArray: cheesItem)
        }
    }
}

in debug I always get nil, but in set method, the newValue is received properly. Could you, please , advice, where is the issue with this?

what is interesting, when apply this approach to standard items like UINavigationController, it works properly.

it only works right for classes (not structs) and on top only for those that are objc compatible.

for a workaround, see also: https://wezzard.com/2015/10/09/associated-object-and-swift-struct/