Ordering a specific post type

Update

I’ve found the solution, by first filtering with is_main_query(), and then using the is_post_type_archive( 'tribe_events' ) conditional statement as was proposed in the first answer.

function order_calendar_by_artist( $query ){    

   if ( is_admin() ||  !$query->is_main_query() ) {
      return;
   }

   if( is_post_type_archive( 'tribe_events' ) ){
      $query->set( 'meta_key', 'artist' );
      $query->set( 'orderby', 'meta_value' );
   } 

}
add_filter( 'pre_get_posts', 'order_calendar_by_artist', 99 );

Old Answer 2

Try the following (found on the plugin forum):

function order_calendar_by_artist( $query ) {
    if ( !empty( $query->tribe_is_multi_posttype ) ) {
        $query->set( 'meta_key', 'artist' );
        $query->set( 'orderby', 'meta_value' ); 
    }
}
add_action( 'pre_get_posts', 'order_calendar_by_artist' );

It appears you need to find the correct $query for the calendar plugin. You could always print_r( $query); to see which queries are running on that page and adjust the conditional accordingly. Might take a while but it’s all I can think of at the mo.

Old answer 1

You could wrap the function code inside a conditional statement like so:

function order_calendar_by_artist( $query ) {
    if ( is_post_type_archive( 'tribe_events' ) ) {
        $query->set( 'meta_key', 'artist' );
        $query->set( 'orderby', 'meta_value' ); 
    }

    return $query;
}
add_filter( 'pre_get_posts', 'order_calendar_by_artist', 99 );

What that should do is only adjust the query on the tribe_events post type archive page. The conditional may need adjusting dependent on where the query needs to take affect. At the moment you’re using the pre_get_posts action which is affecting every single query on the site so you’ll need to hone into a specific area in which you want your function to run (i.e. using a conditional).

You can also use the following to avoid affecting the admin side

if ( is_post_type_archive( 'tribe_events' ) && !is_admin() ) {
    ...
}

I’ve not actually tested this however it should work… (he says)

Failing that you might need to utilise the WP_Query() function and create a new query wherever you need to re-order your posts.

$args = array(
    'post_type' => 'tribe_events',
    'orderby'   => 'meta_value_num',
    'order'     => 'ASC', //or DESC
    'meta_key'  => 'artist'
);
$query = new WP_Query( $args );

if ( $query->have_posts() ) :
    while ( $query->have_posts() ) : $query->the_post();
        //Loop stuff
    endwhile;
endif;
wp_reset_postdata();