query by meta value then date

query by meta value then date

So i have a query like this
function filter_where( $where = '' ) {
// posts in the last 7 days
$where .= " AND post_date > '" . date('Y-m-d',
strtotime('-3 days')) . "'";
return $where;
}
add_filter( 'posts_where', 'filter_where' );
$args = array( 'posts_per_page' => 5,
'meta_key' => 'homepage_toppost',
'orderby' => 'meta_value_num date',
'order' => 'DESC'
);
$popularpost = new WP_Query( $args );
remove_filter( 'posts_where', 'filter_where' );
homepage_toppost is meta key with value from 1 to 5, its just numbers. NOw
i need to show posts ordered by this meta key homepage_toppost then by
date, but its ignoring my meta key and ordering only by date. What am i
doing wrong ?