Post in edit mode defaults author to original author of post.

Unfortunately, I don’t know when this issue started happening. Since I am using a managed WordPress hosting provider it may have been due to the WordPress upgrade to 4.1.1.

Steps to recreate issue:
User A creates a post
Save post (draft or publish, doesn’t matter)
User A edits post and adds a co-author User B
Save post
User A edits post
**BUG** Author shown is User A alone

Results of Debugging
After some debugging, I found the issue was with get_coauthors() from within function coauthors_meta_box. Since get_coauthors() is called in a few places, I presume, without any issues, the problem might be a timing issue when this particular call is made and not having the taxonomy “authors” available yet.

The functions of the call stack are:
co-authors() -> get_the_terms() -> get_object_term_cache() -> wp_cache_get()

get_object_term_cache() returns a WP_error object with an error of “Invalid Taxonomy”.

Workaround Temporary Solution
I bypass the cache by commenting out get_coauthors() from within function coauthors_meta_box and added the following:

$coauthor_terms = wp_get_object_terms( $post->ID, $coauthors_plus->coauthor_taxonomy);

Then follow it by the following code that the get_coauthors() functions does in order to get the output in the appropriate format:

if ( is_array( $coauthor_terms ) && !empty( $coauthor_terms ) ) {
  foreach( $coauthor_terms as $coauthor ) {
    $coauthor_slug = preg_replace( '#^cap\-#', '', $coauthor->slug );
    $post_author =  $coauthors_plus->get_coauthor_by( 'user_nicename', $coauthor_slug );
    // In case the user has been deleted while plugin was deactivated
    if ( !empty( $post_author ) )
      $coauthors[] = $post_author;
  }
} else if ( !$coauthors_plus->force_guest_authors ) {
  if ( $post ) {
    $post_author = get_userdata( $post->post_author );
  } else {
    $post_author = get_userdata( $wpdb->get_var( $wpdb->prepare("SELECT post_author FROM $wpdb->posts WHERE ID = %d", $post_id ) ) );
  }
  if ( !empty( $post_author ) )
    $coauthors[] = $post_author;
} // the empty else case is because if we force guest authors, we don't ever care what value wp_posts.post_author has.

I’m not well versed enough in the inner working of WordPress Core to quickly debug this further, but I hope my workaround solution helps anyone having the same issue.

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