Sort a custom post type loop by a custom taxomomy instead of chronologically

Question

I simply want to list out the books (a custom post type) organized by their respective genre (the taxonomy made with the code below). I have all the code being used listed below.

I am using the following code in a page template to call the custom post type 'book':

<?php
    if ( get_query_var('paged') ) { $paged = get_query_var('paged'); } 
    elseif ( get_query_var('page') ) { $paged = get_query_var('page'); } 
    else { $paged = 1; }
    $myposts = array(
    'post_type' => 'book',
    'posts_per_page' => '40',
    'orderby' => 'id',
    'order' => 'ASC',
    'paged'=> $paged
   );
  query_posts($myposts);
?>

This is the custom post type origin in the functions.php file:

add_action( 'init', 'register_cpt_book' );

function register_cpt_book() {

    $labels = array( 
        'name' => _x( 'Books', 'book' ),
        'singular_name' => _x( 'Book', 'book' ),
        'add_new' => _x( 'Add New', 'book' ),
        'add_new_item' => _x( 'Add New Book', 'book' ),
        'edit_item' => _x( 'Edit Book', 'book' ),
        'new_item' => _x( 'New Book', 'book' ),
        'view_item' => _x( 'View Book', 'book' ),
        'search_items' => _x( 'Search Books', 'book' ),
        'not_found' => _x( 'No books found', 'book' ),
        'not_found_in_trash' => _x( 'No books found in Trash', 'book' ),
        'parent_item_colon' => _x( 'Parent Book:', 'book' ),
        'menu_name' => _x( 'Books', 'book' ),
    );

    $args = array( 
        'labels' => $labels,
        'hierarchical' => true,
        'description' => 'A collection of books',
        'supports' => array( 'title', 'editor', 'excerpt', 'author',  'custom-fields' ),
        'taxonomies' => array( 'author', 'genre' ),
        'public' => true,
        'show_ui' => true,
        'show_in_menu' => true,
        'show_in_nav_menus' => true,
        'publicly_queryable' => true,
        'exclude_from_search' => false,
        'has_archive' => true,
        'query_var' => true,
        'can_export' => true,
        'rewrite' => true,
        'capability_type' => 'post'
    );

    register_post_type( 'book', $args );

}

Here is the custom taxonomy 'genres' in functions.php:

// Add new taxonomy, make it hierarchical (like categories)
    $labels = array(
        'name'              => _x( 'Genres', 'genre' ),
        'singular_name'     => _x( 'Genre', 'genre' ),
        'search_items'      => __( 'Search Genres' ),
        'all_items'         => __( 'All Genres' ),
        'parent_item'       => __( 'Parent Genre' ),
        'parent_item_colon' => __( 'Parent Genre:' ),
        'edit_item'         => __( 'Edit Genre' ),
        'update_item'       => __( 'Update Genre' ),
        'add_new_item'      => __( 'Add New Genre' ),
        'new_item_name'     => __( 'New Genre Name' ),
        'menu_name'         => __( 'Genre' ),
    );

    $args = array(
        'hierarchical'      => true,
        'labels'            => $labels,
        'show_ui'           => true,
        'show_admin_column' => true,
        'query_var'         => true,
        'rewrite'           => array( 'slug' => 'genre' ),
    );

    register_taxonomy( 'genre', array( 'book' ), $args );

The code works great. Like I said, I just want to list out the books organized by their respective genre (the taxonomy made above).

So, the book loop would then look something like this:

Philosophy

Philosophy Book 1, Philosophy Book 2, Philosophy Book 3

Psychology

Psychology Book 1, Psychology Book 2, Psychology Book 3

History

History Book 1, History Book 2, History Book 3

I have tried several methods I found when googling this, but haven't been able to find anything that works. I would be greatly indebted if anyone could give me a workable answer. Thanks.

Answer 1

So it turns out that I didn't do a loop of loops at all. This is several years old now. I don't even know how I came up with that because clauses/limits and DB stuff are still a bit over my head. Anyway, I hope it helps some... I thought for sure I had a loop of loops, but that shouldn't be all that hard to write either.

//remove limits from article CPT archive
function kia_archive_limits( $limits ) {

  if( ! is_admin() && ( is_post_type_archive( 'article' ) ) ) {
     // remove limits
     $limits = '';
  }

  return $limits;
}
add_filter('post_limits', 'kia_archive_limits' );


//complicated queries for articles depending on query var
function kia_article_clauses( $clauses, $wp_query ) {
    global $wpdb;

    if( is_post_type_archive( 'article' ))  {

        //join term_relationships to posts, and term_relationships to term_taxonomy and term_taxonomy to terms
        $clauses['join'] .= "LEFT OUTER JOIN {$wpdb->term_relationships} ON {$wpdb->posts}.ID={$wpdb->term_relationships}.object_id
LEFT OUTER JOIN {$wpdb->term_taxonomy} USING (term_taxonomy_id)
LEFT OUTER JOIN {$wpdb->terms} USING (term_id)" ;

        //group posts by term name
        $clauses['groupby'] = "object_id";

        //include posts with and without a subject term
        $clauses['where'] .= " AND (taxonomy = 'subject' OR taxonomy IS NULL)";
        $clauses['orderby']  = "GROUP_CONCAT({$wpdb->terms}.name ORDER BY name ASC) ";

    } 

    return $clauses;
}
add_filter( 'posts_clauses', 'kia_article_clauses', 10, 2 );

And my loop code looked something like this:

<div class="column">

<?php

$prev = ''; $counter = 1; $column = 1;

while ( have_posts() ) : the_post();  

    $subhead =  array_shift(wp_get_post_terms(get_the_ID(), 'subject', array("fields" => "names")));
    $subhead = $subhead ? $subhead : __('No Subject','peterwade');

    if($counter >= 20) { 
        //reset the counter
        $counter = 1;

        $column++;

        //what column are we on?
        if($column == 5 ){
            $column = 1; //reset column count
            $hr = '<hr class="clear"/>';
        } else { 
            $hr = false; 
        }

        if ($column == 4) { 
            $class = "column last";
        } else {
            $class = "column";
        }

     ?>
    </div><!--.column-->

    <?php if($hr) echo $hr;?>

    <div class="<?php echo $class;?>">

    <h3 class="subhead"><?php printf('%s <span class="continued">%s</span>', $subhead, __('continued'));?></h3>
    <?php 
        }


    if($subhead != $prev) { 
        $counter++; ?>
        <h3 class="subhead"><?php echo $subhead;?></h3>
        <ul>
    <?php } ?>

        <li><a href="<?php echo get_permalink();?>" title="<?php echo __('Permalink to') . ' ' . get_the_title();?>"><?php the_title();?></a></li>
    <?php 

    if($subhead != $prev) { ?>
        </ul>
    <?php }

    $prev = $subhead;
    $counter++;

endwhile; ?>

</div><!--.column-->

Now looking at the source on the page I linked you to, it would appear that he's reverted to how he was doing things formerly.... aka... a giant table. Whether this was because my code stopped working I couldn't say.

Alternative

This would be a loop of loops. Looping through the terms and then querying the posts in each term. You'd have to benchmark to determine which method is faster. I would highly suggested storing the queries in some kind of transient, but it is time for dinner!

$terms = get_terms( 'genre' );

if( $terms ):

foreach( $terms as $term ){

    // The Query
    $args = array(
      'post_type' => 'book',
      'nopaging' => true,
      'tax_query' => array(
         array( 
            'taxonomy' => 'genre',
            'field' => 'id',
            'terms' => $term->term_id 
         )
      )    
);

    $the_query = new WP_Query( $args );

    // The Loop
    if ( $the_query->have_posts() ) {
        echo '<h2>' . ucwords( $term->name ) . '</h2>';
            echo '<ul>';
        while ( $the_query->have_posts() ) {
            $the_query->the_post();
            echo '<li>' . get_the_title() . '</li>';
        }
        echo '</ul>';
    } 
}

endif;

/* Restore original Post Data */
wp_reset_postdata();