Ever wanted to use "WHERE meta_key = value" as a type of sticky?

meta-sticky.php

<?php

/**
 * Use case: Want to order by meta key = value, while including all posts, not just those matching said meta value.
 * 
 * WP_Query's capacity for ordering has improved so much - but as of right now, it's not possible
 * to get _all_ posts, not just ones where a value may or may not exist, and may or may not equal something, and 
 * have all of those be at the top of results.
 * 
 * My very specific use case: a use can set their city and it's saved as a cookie.
 * All products, in this case, have a meta_key.  Simply, city = Name of City.
 * If a user has set their city equal to a city that posts belong to - those posts should show at the top of any results.
 * 
 * Simple enough?
 * 
 * Should probably note: this is currently utterly, utterly untested.
 */ 

add_filter( 'the_posts', function( $posts, $query ) {

	if ( is_admin() ) {
		return $posts;
	}

	if ( 'product' == $query->get( 'post_type' ) ) {

		$user_city = Class::get_user_city();
		$stickies  = array();

		foreach ( $posts as $index => $post ) {

			if ( $post->city && $user_city == $post->city ) {
				$stickies[] = $post;
				unset( $posts[ $index ] );
			}

		}

		$posts = array_values( $stickies + $posts );

	}

	return $posts;

}, 10, 2 );