Custom query vars, rewrite rules and admin columns

lukas.php

/**
 * Make a custom permalink structure on top of a custom post type for yearly archives
 * mapping the year to a meta field
 */



/**
 * Register a custom query variable in our example 'event_year'
 * This can later be used in WP_Query like `get_query_var( 'event_year' )`
 *
 * @param   array   $query_vars     query vars
 * @return  array   $query_vars     query vars
 */
function modifyQueryVars( $query_vars ) {
    $query_vars[] = 'event_year';

    return $query_vars;
}

add_filter( 'query_vars', 'modifyQueryVars' );


/**
 * Register a custom rewrite rules on top of a custom post type for event year archives
 * events/{year}
 */
function modifyRewriteRules() {
    // by year 
    add_rewrite_rule(
        'events/([0-9]{4})/?$',
        'index.php?post_type=events&event_year=$matches[1]',
        'top'
    );
}

add_action( 'init', 'modifyRewriteRules' );




/**
 * Add a custom admin column with data from the `event_year` meta field
 * and make the post list sortable by its values
 */


/**
 * Add a custom column to the events list
 * Note: The name of Wordpress hook we use contains the post type!!!
 *
 * @param   array   $columns    columns of the edit events list
 * @return  array   $columns  
 */
function modifyAdminColumns( $columns ) {  
    $columns[ 'event_date' ] = __( 'Event Date', 'your-text-domain' ); 

    return $columns;  
}  

add_filter( 'manage_edit-events_columns', 'modifyAdminColumns' );  


/**
 * Populate the column with the actual values for each post 
 * Note: The name of Wordpress hook we use contains the post type!!!
 *
 * @param   int     $column_name    name of the columns to render values in.
 * @param   int     $post_id        post ID
 */
function populateCustomAdminColumns( $column_name, $post_id ) {  
    if ( 'event_date' != $column_name ) {
        return;  
    }
    
    // get the value of your field...
    if( $value = get_post_meta( $post_id, 'event-date', true ) ) {   
        echo $value;
    } else {
        echo __( 'None', 'your-text-domain' );
    }

}  

add_action( 'manage_events_posts_custom_column', 'populateCustomAdminColumns', 10, 2 );  


/**
 * Enable sorting the post list by our custom columns
 * Note: The name of Wordpress hook we use contains the post type!!!
 *
 * @param   array   $columns    columns of the edit events list
 * @return  array   $columns
 */
function ModifySortableAdminColumns( $columns ) {  
    $columns[ 'event_date' ] = 'event_date';  
    
    return $columns;  
}  

add_filter( 'manage_edit-events_sortable_columns', 'ModifySortableAdminColumns' );