Loop that displays the first post of every available custom post type?

Question

Is it possible to create a loop that goes through all the available custom post types without statically defining what those custom post types are, and then displaying the featured image of the first post for each of those custom post types?

Answer 1

This should work for you:

// grab all public post types
$post_types = get_post_types( array('public' => true), 'names' );

// loops through each post type
foreach( $post_types as $type ) {

    // setup the query
    $query_args = array(
        'post_type' => $type,
        'posts_per_page' => 1
    );

    // perform the query
    $items = get_posts( $query_args );

    // check if we have found anything
    if( $items ) {

        // loop through the items
        foreach( $items as $item ) {

            // show the post thumbnail
            echo get_the_post_thumbnail( $item->ID, 'thumbnail' );

        }

    }

}

Answer 2

As an addition to @Pippin answer:

It is even possible to use a single query, with a slightly different syntax.

// Get all post types
$post_types = get_post_types( 
     array( 
         'public'       => true
         // Avoid attachments - those aren't shown in the admin menu
        ,'show_in_menu' => true 
     )
    ,'names'
);

// Get rid of unwanted post types
// Use the array to add your unwanted post types
foreach ( array( 'some_unwanted_post_type', 'another_one' ) as $not_me )
    in_array( $not_me, $post_types ) AND unset( $post_types[ $not_me ] );

// Group by filter
add_filter( 'posts_groupby', 'wpse57806_posts_groupby', 20 );

// Query posts - Two options for post_types
$items = get_posts( array(
     'post_type'      => $post_types
    // ... or ...
    # 'post_type'       => 'any' // every post type, but not attachments
    ,'posts_per_page' => count( $post_types )
) );

// Call the thumbnail
foreach ( $items as $item )
    echo get_the_post_thumbnail( $item->ID, 'thumbnail' );

Edit

You'll need a small callback function to make the edit work:

function wpse57806_posts_groupby()
{
    // only needed once - better kept in here
    remove_filter( current_filter(), __FUNCTION__ );

    return 'post_type';
}