display one post out of each custom category (aka "term of custom taxonomy")

Question

I would like to display one post (oldest, but that's not that important for now) out of each of the custom category except one (called 'home').

So far I got to

1) display each custom taxonomy slug existing except one that I don't need.
2) display all custom posts of a certain taxonomy ('portraits')

How do I do the third and main part of the script - display one post from each of the custom taxonomy?

  //get all custom categories (taxonomy)
$terms = get_terms(array('taxonomy' => 'album', 'exclude' => 4)); 
foreach ($terms as $term){
    echo $term->slug; 
    echo "<br/>"; 
}
//get all custom post types of custom category "portraits"
//however, need to get one oldest post of each custom category from above
    $args = array(
    'post_type' => 'simple_image', 
    'posts_per_page' => '-1',
    'order_by' => 'date', 
    'order' => 'ASC', 
    'tax_query' => array(
        array(
            'taxonomy' => 'album',
            'field' => 'slug',
            'terms' => 'portraits' 
        )
    )
);

 $new_query = new WP_Query ($args);
  while ( $new_query->have_posts() ) : $new_query->the_post();    
         the_post_thumbnail('thumbnail') ; 
         endwhile; 

And why I am doing it - it's a photo gallery theme and I want to display a list of albums with a thumbnail. The thumbnail will come from one of the posts in the album.

[edit] I've seen a script where WP query was put in the foreach loop, that doesn't make much sense to me. Is there something simpler? Isn't there JOIN operation?

Answer 1

You can use tax_query to query your posts based on the taxonomy. But be advised, if you have a lot of categories, then this is gonna take a lot of resources, unless you use a cache plugin.

$terms = get_terms(array('taxonomy' => 'album', 'exclude' => 4)); 
foreach ($terms as $term){
    $args = array(
    'post_type' => 'simple_image', 
    'posts_per_page' => '1',
    'order_by' => 'date', 
    'order' => 'ASC', 
    'tax_query' => array(
        array(
            'taxonomy' => 'album',
            'field' => 'slug',
            'terms' => $term->slug, 
            )
        )
    );
    $new_query = new WP_Query ($args);
    if ($new_query->have_posts()) {
        while($new_query->have_posts()){
            $new_query->the_post();
            the_post_thumbnail('thumbnail');
        }
    }
    wp_reset_postdata();
}

As you have already noticed, this will run 100 queries, if you have 100 terms under that taxonomy. So, make sure you use it in conjunction with a cache plugin.