Allow post author to preview non-published post

allo-preview-to-author.php

<?php
/**
 * Allow author to preview non published solution.
 *
 * @param  object $posts
 * @return object
 */
function preview_post_to_author( $posts ) {

    // Get surrent user id.
    $current_user_id = get_current_user_id();

    // Bail, if non-logged in.
    if ( empty( $current_user_id ) ) {
        return $posts;
    }

    global $wp_query, $wpdb;

    $post_id = $wp_query->query['p'];

    // Bail, if no post id found.
    if ( empty( $post_id ) ) {
        return $posts;
    }

    // Get current post author id.
    $author_id = get_post_field( 'post_author', $post_id );

    // Bail, if no author found.
    if ( empty( $author_id ) ) {
        return $posts;
    }

    // Set result for non-adminstrator and solution author.
    if ( is_single() &&
         $wp_query->post_count == 0 &&
         $author_id == $current_user_id &&
         ! current_user_can( 'adminstrator' ) ) {

        $posts = $wpdb->get_results( $wp_query->request );

    }

   return $posts;

}

add_filter( 'the_posts', 'preview_post_to_author' );