I can’t set meta_key in my custom post type query

I’ve created a custom post, events, and saved the meta data event_year to each event. I’m trying to get a year’s event archive. When I go the the following URL, the meta_key and meta_value are not set for some reason, and thus the event archive is unfiltered.

/?post_type=events&meta_key=event_year&meta_value=2011

Debugging the values:

echo $wp_query->query_vars['post_type']; // 'events'
echo $wp_query->query_vars['meta_key']; // -blank-
echo $wp_query->query_vars['meta_value']; // -blank-

Why can’t I set the meta_key and meta_value?

The meta_values are saved to the events. I’ve successfully displayed them:

get_post_meta($post->ID, 'event_year', true); // '2011'

Answers:

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Method 1

That’s because ‘meta_key’ and ‘meta_value’ are not public query vars. In other words, you can’t use them in URLs directly, nor should you.

Instead, register a specific query var, like so:

function register_my_qv() {
  global $wp;
  $wp->add_query_var( 'my_qv' );
}
add_action( 'init', 'register_my_qv' );

Then, you can go to a URL like this: ?my_qv=foobar

All you need to do now is map your query var to the actual query you want to do:

function map_my_qv( $wp_query ) {
  if ( $meta_value = $wp_query->get( 'my_qv' ) ) {
    $wp_query->set( 'meta_key', 'some_meta_key' );
    $wp_query->set( 'meta_value', $meta_value );
  }
}
add_action( 'parse_query', 'map_my_qv' );

Method 2

I can’t add a comment to the scribu’s excellent answer due to low reputation, but still the second part of the code (map_my_qv function) while working on WP 4.2 was giving me 404s, missing posts in admin and PHP notices about $meta_value variable not set. Therefore, here’s the edited code:

function map_my_qv( $wp_query ) {
    if ( is_admin() || ! $wp_query->is_main_query() ) 
        return; 

    if ( $wp_query->get( 'my_qv1' ) ) {
        $wp_query->set( 'meta_key', 'my_meta_key1' );
        $wp_query->set( 'meta_value', $wp_query->get( 'my_qv1' ) );
    }
    if ( $wp_query->get( 'my_qv2' ) ) {
        $wp_query->set( 'meta_key', 'my_meta_key2' );
        $wp_query->set( 'meta_value', $wp_query->get( 'my_qv2' ) );
    }
}
add_action( 'parse_query', 'map_my_qv' );

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0