Wordpress: Automatically sets the post thumbnail

functions.php

<?php
/**
 * Automatically sets the post thumbnail.
 *
 * @global array $post WP post object.
 */
function odin_autoset_featured() {
    global $post;

    if ( isset( $post->ID ) ) {
        $already_has_thumb = has_post_thumbnail( $post->ID );
        if ( ! $already_has_thumb ) {
            $attached_image = get_children( 'post_parent=' . $post->ID . '&post_type=attachment&post_mime_type=image&numberposts=1' );
            if ( $attached_image ) {
                foreach ( $attached_image as $attachment_id => $attachment ) {
                    set_post_thumbnail( $post->ID, $attachment_id );
                }
            }
        }
    }
}

add_action( 'the_post', 'odin_autoset_featured' );
add_action( 'save_post', 'odin_autoset_featured' );
add_action( 'draft_to_publish', 'odin_autoset_featured' );
add_action( 'new_to_publish', 'odin_autoset_featured' );
add_action( 'pending_to_publish', 'odin_autoset_featured' );
add_action( 'future_to_publish', 'odin_autoset_featured' );