Ambrosite Next/Previous Post Link Plus or WP next_post_link to cycle through top level category

Question

I have installed Ambrosite Next/Previous Post Link Plus and I'm not sure how to configure it to achieve what I'm looking for.

I have a post category like below:

Parent category -> Sub category -> Post

If I use the traditional WP next_post links I can use TRUE or FALSE to cycle through either all posts on the site or the posts in the sub category.

How can I configure either Ambrosite Next/Previous Post Link Plus or the standard WP links to cycle through the posts that belong to the parent category and not just the sub category?

Answer 1

That should be default behavior, provided that the checkbox of the parent category was checked. From the codex: "If the post is in both the parent and subcategory, or more than one category, the next post link will lead to the next post in any of those categories."

Answer 2

1. Put following function in functions.php. It adds the IDs of the (sub)categories and their parents to an array, removes duplicates and returns the array:

   function wpse80138_get_cat_ids( $post_id )
   {
       $array = array();
       $cats = get_the_category( $post_id );
       foreach( $cats as $cat ) {
           $array[] = $cat->term_id;
           // parents
           if( $cat->category_parent ) 
               $array[] = $cat->category_parent;
           // children
           $subs = get_categories( 'child_of=' . $cat->term_id );
           foreach( $subs as $sub )
               $array[] = $sub->term_id;
       }
       return implode( ',', array_unique( $array ) );
   }
   

2. Use the function with Ambrosite:

   $cats = wpse80138_get_cat_ids( get_the_ID() );
   next_post_link_plus( array( 'in_cats' => $cats ) );