using posts_where for meta data on pre_get_posts

Let's say you have some code which filters posts by using pre_get_posts

e.g.

add_action( 'pre_get_posts', 'be_exclude_category_from_blog' );

function be_exclude_category_from_blog( $query ) {

    if( $query-is_main_query()  $query-is_home() ) {
        $query-set( 'cat', '-4' );
    }

}

I found a nice bit of code which enables you to further filter these posts to the last 90 days:

add_action( 'pre_get_posts', 'be_exclude_category_from_blog' );

function be_exclude_category_from_blog( $query ) {

    if( $query-is_main_query()  $query-is_home() ) {
        add_filter( 'posts_where', 'filter_date' );
        $query-set( 'cat', '-4' );
    }

}

function filter_date( $where = '' ) {
    $where .= " AND post_date  '" . date('Y-m-d', strtotime('-90 days')) . "'";
    return $where;
}

This works really well. But what if I want to use the same method to target a post's meta data instead of just post_date? Is this possible? From this answer it looks like you should do something like this:

add_action( 'pre_get_posts', 'be_exclude_category_from_blog' );

function be_exclude_category_from_blog( $query ) {

    if( $query-is_main_query()  $query-is_home() ) {
        add_filter( 'posts_where', 'filter_date' );
        $query-set( 'cat', '-4' );
    }

}

function filter_date( $where = '' ) {
    global $wpdb;  
    $where .= " AND ($wpdb-postmeta.meta_key = '_length' AND $wpdb-postmeta.meta_value = '61')";
    return $where;
}

However this does not work for me, it returns zero results. Even though I know there are posts with a meta key of _length with a value of 61.

Could someone point me in the right direction?

Cheers