Query all posts with specific tag

Question

I need to fetch all the posts with a specific tag, but I'm getting all the posts instead. My query works if I publish a post with the tag that I need and list all the posts with that tag, but when I publish a post with another tag, it's fetching the newly published post.

This is my query:

$original_query = $wp_query;
$wp_query = null;
$args=array(
    'posts_per_page' => -1, 
    'tag' => $post_tag
);
$wp_query = new WP_Query( $args );
$post_titles=array();
$i=0;
if ( have_posts() ) :
    while (have_posts()) : the_post();
        $post_titles[$i]=get_the_ID() ;
        $i++;
    endwhile;
endif;
$wp_query = null;
$wp_query = $original_query;
wp_reset_postdata();

Answer 1

It's a lot easier to create a new WP_Query than it is to try clearing or overwriting the original.

If $post_tag is a tag slug, you could simply use:

<?php
$the_query = new WP_Query( 'tag='.$post_tag );

if ( $the_query->have_posts() ) {
    echo '<ul>';
    while ( $the_query->have_posts() ) {
        $the_query->the_post();
        echo '<li>' . get_the_title() . '</li>';
    }
    echo '</ul>';
} else {
    // no posts found
}
/* Restore original Post Data */
wp_reset_postdata();

Answer 2

In your functions.php

/* Display Related Products */
/* ======================== */

if ( ! function_exists( 'display_related_products' ) ) {

    function display_related_products($post_tag) {
        ?>
        <div class="related-products">

            <!-- simple WP_Query -->
            <?php
                $args = array(
                    'post_type' => 'product',
                    'tag' => $post_tag, // Here is where is being filtered by the tag you want
                    'orderby' => 'id',
                    'order' => 'ASC'
                );

                $related_products = new WP_Query( $args );
            ?>

            <?php while ( $related_products -> have_posts() ) : $related_products -> the_post(); ?>

                <a href="<?php the_permalink(); ?>" class="related-product">
                    <?php if( has_post_thumbnail() ) : ?>
                        <?php the_post_thumbnail( 'full', array( 'class' => 'related-product-img', 'alt' => get_the_title() ) ); ?>
                    <?php endif; ?>
                </a>

            <?php endwhile; wp_reset_query(); ?>

        </div>
        <?php
    }
}

Call from anywhere with

display_related_products('name-of-the-tag');