Resetting post data to previous loop in nested loops

I’m trying to use nested loops with the posts to posts plugin. The loops both work, but the problem arises after the second nested loop ($issue). I want to access the $publication loop again, but the data is still the $issue data.

wp_reset_query() will reset right back to the main loop in single.php which I don’t want.

I could use get_posts() instead of new WP_Query, but I want to be able to use get_template_part().

How can I reset my data back to the publication loop, so that the second ‘Publication title’ returns the publication, not the issue, title?

Here’s my code within single.php:

$publication = new WP_Query( array(
'connected_type'  => 'publication_to_post',
'connected_items' => $post->ID,
'fields'          => 'ids',
'posts_per_page'  => 1,
) );

if ( $publication->have_posts() ) {
while ( $publication->have_posts() ) : $publication->the_post();
    echo '<h2>Publication title=".get_the_title()."</h2>';
    $pub_id = get_the_ID();

    $issue = new WP_Query( array(
        'connected_type'  => 'publication_to_issue',
        'connected_items' => $pub_id,
        'fields'          => 'ids',
        'posts_per_page'  => 1,
    ) );

    if ( $issue->have_posts() ) {
        while ( $issue->have_posts() ) : $issue->the_post();

            // need to be able to use template parts in here
            echo '<h2>Issue title=".get_the_title()."</h2>';

        endwhile;
    }

    // This currently returns the issue title, not the publication title
    echo '<h2>Publication title=".get_the_title()."</h2>';

endwhile;
}

I’m going to answer this myself, but it was the very clever @simonwheatley of Code for the People that solved this one for me.

Instead of using wp_reset_postdata() or wp_reset_query(), you can use the following:

$publication->reset_postdata();

Where $publication is your query object.

The working code now looks like:

$publication = new WP_Query( array(
'connected_type'  => 'publication_to_post',
'connected_items' => $post->ID,
'fields'          => 'ids',
'posts_per_page'  => 1,
) );

if ( $publication->have_posts() ) {
while ( $publication->have_posts() ) : $publication->the_post();
    echo '<h2>Publication title=".get_the_title()."</h2>';
    $pub_id = get_the_ID();

    $issue = new WP_Query( array(
        'connected_type'  => 'publication_to_issue',
        'connected_items' => $pub_id,
        'fields'          => 'ids',
        'posts_per_page'  => 1,
    ) );

    if ( $issue->have_posts() ) {
        while ( $issue->have_posts() ) : $issue->the_post();

            // need to be able to use template parts in here
            echo '<h2>Issue title=".get_the_title()."</h2>';

        endwhile; $publication->reset_postdata();
    }

    echo '<h2>Publication title=".get_the_title()."</h2>';

endwhile;
}