How to change main query based on post meta

Question

I would like to change the sort order of a query, based on meta in the returned posts. Specifically, posts are events with an event date and I filter them as either future or past. If all events are in the past, I want the most recent ones first, if all in the future, the next to happen needs to be first. I've tried all sorts in a pre-get-posts function like:

function resort($query){
    if($query->is_main_query() && is_tax('event-category'){
        if($query->have_posts()){
            // do something based on the first post
            wp_reset_query();
        }
    }
}

The function is in functions.php and is firing but it doesn't get past have_posts(), even though the main query is returning lots of posts. What am I missing?

1. What is it that triggers the query, and can't it be triggered anywhere? I tried query_posts() and failed.
2. How can I construct a custom query which is exactly the main query?
3. expense doesn't bother me - it's a small site.

Answer 1

Finally got it. This code in functions.php:

function set_order_by_last_date($query){

$query->set( 'orderby', 'eventstart');
$query->set( 'order', 'DESC' );
$query->get_posts();
if ($query->have_posts()){
    if ( eo_get_the_end('Ymd',$query->posts[0]->ID,$query->posts[0]->occurrence_id) > date('Ymd') ){
        $query->set( 'event_end_after', 'today' );
        $query->set( 'orderby', 'eventstart');
        $query->set( 'order', 'ASC' );
    } else {
        $query->set( 'event_end_before', 'today' );
        $query->set( 'orderby', 'eventstart');
        $query->set( 'order', 'DESC' );
    }
}
return $query->query_vars;
}

Called from the template file with:

<?php query_posts(set_order_by_last_date(clone $wp_query));?>

The vars and eo_ functions are from the excellent event organiser plug-in. Thanks @jack-johansson for the pointers.