upgrade.php

<?php
function coauthors_plus_upgrade( $from ) {
	// TODO: handle upgrade failures
	
	if( $from < 2.0 ) coauthors_plus_upgrade_20();
}

/** 
 * Upgrade to 2.0
 * Updates coauthors from old meta-based storage to taxonomy-based
 */
function coauthors_plus_upgrade_20 () {
	global $coauthors_plus;
	
	// Get all posts with meta_key _coauthor
	$all_posts = get_posts(array('numberposts' => '-1', 'meta_key' => '_coauthor'));
	
	if(is_array($all_posts)) {
		foreach($all_posts as $single_post) {
			
			// reset execution time limit
			set_time_limit( 60 );
			
			// create new array
			$coauthors = array();
			// get author id -- try to use get_profile
			$coauthor = get_user_by( 'id', (int)$single_post->post_author );
			if ( is_object( $coauthor ) )
				$coauthors[] = $coauthor->user_login;
			// get coauthors id
			$legacy_coauthors = get_post_meta($single_post->ID, '_coauthor'); 
			
			if(is_array($legacy_coauthors)) {
				//echo '<p>Has Legacy coauthors';
				foreach($legacy_coauthors as $legacy_coauthor) {
					$legacy_coauthor_login = get_user_by( 'id', (int)$legacy_coauthor );
					if ( is_object( $legacy_coauthor_login ) && ! in_array( $legacy_coauthor_login->user_login, $coauthors ) ) $coauthors[] = $legacy_coauthor_login->user_login;
				}
			} else {
				// No Legacy coauthors
			}
			$coauthors_plus->add_coauthors($single_post->ID, $coauthors);
			
		}
	}
}