Show only one category in main query, issues on tag page

Question

I altered the following code from Ashfame:

add_action('pre_get_posts', 'block_cat_query' );
function block_cat_query() {
global $wp_query;   
if( is_home() || is_search() || is_tag() ) {
    $wp_query->query_vars['cat'] = '3';
}
}

This code DOES work to filter posts from category #3 only on the home, search and tag pages.

However, I am utilizing

<?php single_tag_title(); ?>

in my header.php file and on specific tag pages, the tag title is not showing up. The page is instead showing the category title. This is only happening with this function in place, so I'm trying to figure out if there is another way of writing the function above that does not effect the use of <?php single_tag_title(); ?> on tag pages.

Answer 1

If the only issue you want to correct is the output of single_tag_title, you can correct the value via a filter by grabbing the tag query var directly:

function wpa90852_fix_tag_title(){
    $tag = get_term_by( 'slug', get_query_var('tag'), 'post_tag' );
    return $tag->name;
}
add_filter( 'single_tag_title', 'wpa90852_fix_tag_title' );

Answer 2

First you have to find the right hook, here it is pre_get_posts. Then you have to check if you are on the front page (is_front_page()) and if you are in the main query (is_main_query). As the $query is given as a parameter it's easy to alter the query.

/**
 * WordPress Answers #90852
 *
 * @param obj $query WP_Query object
 * @return void
 * @author Ralf Hortt
 **/
function pre_get_posts_90873( $query )
{
    if ( is_front_page() && $query->is_main_query() && !is_admin() ) :
        $query->query_vars['cat'] = 3;
    endif;
}
add_action('pre_get_posts', 'pre_get_posts_90873' );

PS: Ok, I'll see the question has changed. I still let my answer her, just in case someone want to know.

Answer 3

There're a lot of methods for the WP_Query that you can use to achieve your goal. If you want to block a category on:

• The home page
• The Search-results page
• On tag pages

by its ID, then you can use category__not_in.

add_filter( 'pre_get_posts', 'wpse90852_block_cat_query' );
function wpse90852_block_cat_query( $query )
{
    if ( is_admin() )
        return $query;

    if ( $query->is_main_query() && ( $query->is_home() || $query->is_search() || $query->is_tag() ) )
    {
        $query->set( 'category__in', 3 );
    }

    return $query;
}