Fix get_the_terms Not Returning Data After Using pre_get_posts on Tax Archive Pages

When you use the pre_get_posts action in WordPress, you’re modifying the main query before it’s executed. This is powerful for altering what posts are displayed on archive pages, search results, or the main blog page. However, it can sometimes interfere with functions like get_the_terms() if the context for retrieving terms is lost or misinterpreted.

The common reasons get_the_terms() might fail on a taxonomy archive page after pre_get_posts are:

1. Incorrect Context for get_the_terms(): get_the_terms() is designed to retrieve terms associated with a specific post. If you’re trying to get information about the archive’s queried term (e.g., the category or tag itself), get_the_terms() is the wrong function.
2. Global $post Object Issues: While pre_get_posts modifies the query, sometimes the global $post object, which get_the_terms() relies on (if no post ID is provided), isn’t correctly set up or is reset prematurely, especially if you’re running custom queries before the main loop.

Solutions

Let’s address both scenarios: getting the queried archive term and ensuring get_the_terms() works for individual posts within the loop.

1. Getting the Queried Taxonomy Term (The Archive’s Term)

If you want to display the name, description, or other details of the taxonomy term that the current archive page is displaying (e.g., “Category: Photography” and its description), you should use get_queried_object(). This function correctly identifies the term object that the current archive page is for, regardless of pre_get_posts modifications.

Example Usage (in archive.php, category.php, tag.php, or taxonomy.php):

<?php
// Get the currently queried object (which will be the term object on a taxonomy archive)
$queried_object = get_queried_object();

if ( $queried_object && is_a( $queried_object, 'WP_Term' ) ) {
    $term_name = $queried_object->name;
    $term_description = $queried_object->description;
    $term_taxonomy = $queried_object->taxonomy; // e.g., 'category', 'post_tag', 'my_custom_taxonomy'

    echo '<h1>' . esc_html( $term_name ) . '</h1>';
    if ( $term_description ) {
        echo '<div class="archive-description">' . wp_kses_post( $term_description ) . '</div>';
    }
}
?>

2. Ensuring get_the_terms() Works for Posts Within the Loop

Inside the standard WordPress loop, get_the_terms() should typically work without issues because the_post() sets up the global $post object for each post. If you’re encountering problems here, it’s less likely to be a direct pre_get_posts conflict and more likely related to:

• Not calling the_post(): Ensure you are correctly iterating through the loop using while ( have_posts() ) : the_post();.
• Running a custom query and not resetting post data: If you’ve run a WP_Query before your main loop and haven’t called wp_reset_postdata(), it can interfere with the main query’s context.

Correct Usage within the Loop:

<?php
if ( have_posts() ) :
    while ( have_posts() ) : the_post();
        // Inside the loop, get_the_terms() should work for the current post
        $post_id = get_the_ID(); // Or simply omit the ID, as it defaults to the current post

        $terms = get_the_terms( $post_id, 'category' ); // Replace 'category' with your taxonomy slug

        if ( $terms && ! is_wp_error( $terms ) ) {
            echo '<ul>';
            foreach ( $terms as $term ) {
                echo '<li>' . esc_html( $term->name ) . '</li>';
            }
            echo '</ul>';
        }

        the_title( '<h2>', '</h2>' );
        the_content();

    endwhile;
    the_posts_pagination(); // Display pagination
else :
    // No posts found
    get_template_part( 'template-parts/content', 'none' );
endif;

// IMPORTANT: If you ran any custom WP_Query before this, ensure you reset post data.
// wp_reset_postdata();
?>

Example of pre_get_posts (for context, not the fix itself)

This example shows how pre_get_posts might be used in your functions.php to modify a taxonomy archive. The fix for get_the_terms is not in this pre_get_posts function, but in how you use get_the_terms in your template files.

<?php
/**
 * functions.php
 */

function my_custom_pre_get_posts_query( $query ) {
    // Check if it's the main query and a taxonomy archive
    if ( ! is_admin() && $query->is_main_query() && $query->is_tax() ) {
        // Example: Only show posts from a specific custom post type on a taxonomy archive
        // $query->set( 'post_type', 'my_custom_post_type' );

        // Example: Order posts by title ascending
        // $query->set( 'orderby', 'title' );
        // $query->set( 'order', 'ASC' );

        // Example: Exclude specific categories from a category archive
        // if ( $query->is_category() ) {
        //     $query->set( 'category__not_in', array( 5, 12 ) ); // Exclude categories with IDs 5 and 12
        // }
    }
}
add_action( 'pre_get_posts', 'my_custom_pre_get_posts_query' );

Summary

The key to resolving get_the_terms issues on taxonomy archive pages after pre_get_posts is to:

• Use get_queried_object() to retrieve details about the archive’s taxonomy term.
• Ensure get_the_terms() is used within the WordPress loop for individual posts, and that the_post() is called to set up the global $post object correctly.
• Always call wp_reset_postdata() if you’ve run a secondary WP_Query before the main loop.

By following these guidelines, you should be able to correctly retrieve taxonomy data on your archive pages.