Using URL parameters, list posts from category and custom taxonomy

In general circumstances, using URL parameters, you can list posts that belong to a specific category AND a custom taxonomy, like this:

http://example.com/category/cars/?edition=usa

Where, category is the Category base you are using for categories on your site (WordPress Dashboard > Settings > Permalinks > Category base); edition is the custom taxonomy’s base/slug; and usa is a term under the custom taxonomy.

If you want to include more than one category/custom taxonomy, these might help:

http://example.com/category/cars/?edition=usa,india
http://example.com/?category_name=cars,books&edition=usa,india

And Feeds:

http://example.com/category/cars/?edition=usa,india&feed=rss2
http://example.com/?category_name=cars,books&edition=usa,india&feed=rss2

Further Reading:

• http://codex.wordpress.org/WordPress_Feeds

• http://codex.wordpress.org/Class_Reference/WP_Query

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BUT…

As explained in my question, mine is a complex case, so I’ve developed a simple work-around. Here goes…

1. The code block in the question makes sure that if a post is unassigned to any Edition (i.e. if a post is not assigned to any term that belongs to the custom taxonomy ‘edition’), the post is shown/listed under all term archives/feeds of the custom taxonomy ‘edition’.

   BUT NOW, I removed that code. Then created a new term under the custom taxonomy ‘edition’, called ‘intl’ (International). Any post that want to be displayed under all Editions will be assigned to ‘intl’. But how do I make sure that all posts assigned to ‘intl’ show up in all term archives/feeds of my custom taxonomy?

   For that, I now use this code (goes in functions.php):

   add_filter('pre_get_posts','better_editions_archive');
   function better_editions_archive( $query ) {
       if ( $query->is_tax( 'edition' ) && $query->is_main_query() ) {
           $query->set( 'post_type', array( 'post' ) );
           $query->set( 'tax_query',
               array(
                   'relation' => 'OR',
                   array(
                       'taxonomy' => 'edition',
                       'field' => 'slug',
                       'terms' => 'intl',
                       'operator' => 'IN'
                   )
               )
           );
       }
       return $query;
   }
   

   So now, for instance, http://example.com/edition/usa/ lists posts that belong to either ‘usa’ or ‘intl’ (which are terms under my custom taxonomy ‘edition’). Its feed, http://example.com/edition/usa/feed/ does the same as well.

2. Back to the main problem of the question. How do I list posts that belong to a specific category AND an edition, using URL parameters?

   For example, how do I list posts that belong to category ‘cars’ AND edition ‘usa’?

   This is what the URL should look like: http://example.com/category/cars/?edition=usa,intl (since we also want those posts that are shown under all editions i.e. those posts assigned to term ‘intl’). As for feeds: http://example.com/category/cars/feed/?edition=india,intl

That’s it!

(Special thanks to @kaiser for his help.)

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Notes

If you strictly want to modify the main query/loop right within the template, e.g. taxonomy-edition.php (in my case), here’s an example as to how it can be done:

<?php
$edition_term = get_term( get_queried_object(), 'edition' )->slug;
$better_editions = new WP_Query(
    array(
        'post_type' => 'post',
        'tax_query' => array(
            array(
                'taxonomy' => 'edition',
                'field' => 'slug',
                'terms' => array( $edition_term, 'intl' )
            )
        )
    )
);
?>

    <?php /* Blah, Blah, Blah! */ ?>

<?php if ( $better_editions->have_posts() ) : ?>

    <?php /* Start the Loop */ ?>
    <?php while ( $better_editions->have_posts() ) : $better_editions->the_post(); ?>
        <?php get_template_part( 'content', get_post_format() ); ?>
    <?php endwhile; wp_reset_postdata(); ?>

<?php else : ?>

    <?php /* Blah, Blah, Blah! */ ?>

<?php endif; ?>

Nonetheless, unless you really, really have to, go with pre_get_posts.