Add a category on all archive category pages

Question

I wonder if it is possible to have the default category WordPress on all category archives?

Here is my situation: I created a website of video games and I renamed the default category "multiplatform". But I want all posts in this category are displayed on other categories like the category "PS3" for example.

If possible, could you help me to access this feature?

Edit: Here's a little example to explain it:

Currently:

• The archive of the "Android" category displays only the items with the category "Android" assigned.
• Assign a post to the "multiplatform" category will only appear on the archive section of posts and the archive of the category "multiplatform" page.

What I want:

• Assign a post to the "multiplatform" category will appear on the archive section of the category "Android" as well as all other categories.

Answer 1

You need to use pre_get_posts hook. Where u can include the default category posts on other category pages.

function wpse_pre_get_posts( $q )
{
    // its the main query, and its the category page archive
    if( $q->is_main_query() && $q->is_category() ) :

        // get the default selected category id, fallback to 1 (default one id)
        $default_cat = (int) get_option('default_category', 1);

        // if the category page is not the default category page
        if( $default_cat != $q->get_queried_object_id() ):

            // get the already used tax_query arguments
            $tax_query = $q->get('tax_query');

            // add the default one
            $tax_query[] = array(
                'taxonomy' => 'category', 
                'field'    => 'term_id', 
                'terms'    => $default_cat 
            );

            // relation should be or, which means either the current category or the default
            $tax_query['relation'] = 'OR';

            // set the values to query
            $q->set('tax_query', $tax_query );

        endif;

    endif;
}
add_action( 'pre_get_posts', 'wpse_pre_get_posts' );

Ok, now this might conflict wp_query variable, so the next solution would be using post_request filer, which is used to filter the raw query.

function wpse_posts_request( $request, $q )
{
    global $wpdb;

    // its the main query, and its the category page archive
    if( $q->is_main_query() && $q->is_category() ) :

        // get the default selected category id, fallback to 1 (default one id)
        $default_cat = (int) get_option('default_category', 1);

        // category now
        $cat_now = $q->get('cat');

        // if the category page is not the default category page
        if( $default_cat != $cat_now ):

            $request = str_replace("$wpdb->term_relationships.term_taxonomy_id IN ($cat_now)", "$wpdb->term_relationships.term_taxonomy_id IN ($cat_now, $default_cat)", $request);

        endif;

    endif;

    return $request;
}
add_filter( 'posts_request', 'wpse_posts_request', 10, 2 );

That's it. You are all done

Answer 2

You can use the bulk edit feature to add a category to all posts in an existing category. Use the filter by category feature.

Answer 3

You can assign your posts to multiple categories. Just check the checkbox before the categories you want your posts to be assigned in post editor page to and your posts will be shown in all the category archives you selected.

Just to let you know, when you assign your post to a category, it won't be automatically assigned to your default category unless you manually select that category.