Wordpress posts order by custom field

Question

I have 2 types of posts - event and news. for event category, i have a custom filed - event_date. but news does not have that field. i want to order the posts according to the event_date (i want to include the news too even though they dont have the event_date field ). i have tried with this:

$args = array(
            'cat'               => $categories,
            'posts_per_page'    => $display_count,
            'post_type'         => 'post',
            'number'            => $display_count,
            'page'              => $page,
            'offset'            => $offset,
            'meta_key'          => 'event_date',
            'orderby'           => 'meta_value_num',
            'order'             => 'ASC'
        );
        $query = new WP_Query($args);

this query does not return any of the news. is this because the news dont have the order field? if so, then how can i solve this problem? i was thinking of ordering with multiple like-

'orderby'=> 'meta_value_num date'

but no luck.