The parent of the content will always be the first category relation. If you want to construct a breadcrumb or similar based on another categorization I can't think of any other way than somehow using the current URL.
I came to the same conclusion. Use the current URL and search for a matching relation. However, I am not sure it is the most beautiful solution.
I have a product in my catalog with a relation, such that the product is present in two different categories. I have then made a breadcrumb by getting the ancestors of the currentContent, but this always have the original category.
I have product A in category B (the original location of A) and C. When seeing product A the breadcrumb will always say B/A also on the URLl “C/A” the breadcrumb will still be B/A.
How can I show the correct breadcrumb when using URL “C/A”?