Add meta_query to wp_site_query

[spacedmonkey]

Currently site query doesn't support meta query as there is no blogmeta table. However with it being added #37923 this functionally can be supported.

Along with adding support this functionality, we should start to use it in a number of place. A key example is adding the ability to search by site title in the network admin panel

[flixos90]

I suggest continuing with the site meta query functionality in #37923 for now, as we would like to work on the entire site meta feature in the format of a feature project (hosted at ​https://github.com/jeremyfelt/wordpress-develop).

When we get to committing, it probably makes sense to break things down to smaller parts, and then this sub-ticket here can come in handy. For now let's keep the discussion going in a centralized location (#37923).

[flixos90]

These tickets belong to our planned roadmap (a few of them not per final decision), so flagging with a keyword for better overview.

[flixos90]

40229.diff​ implements meta query functionality in WP_Site_Query:

• The implementation itself is trivial and straightforward. It follows mostly how other query classes do it. The orderby-meta functionality deserves an extra look because it's the most complicated part, but the provided tests confirm it works as expected.
• class-wp-meta-query.php is loaded earlier in wp-settings.php to allow this functionality early during the multisite bootstrap process. This will fix #40948.
• Comprehensive tests have been added for the meta query functionality, i.e. one parameterized test with 10 datasets, each testing different behavior.
• Three additional tests have been added to verify that adding/updating/deleting site meta busts the site query cache.

[flixos90]

In 43009:

Bootstrap: Load class-wp-meta-query.php earlier in wp-settings.php.

This change is necessary to enable multisite-specific meta query functionality during the multisite bootstrap process.

Fixes #40948. See #40229.

[flixos90]

In 43010:

Multisite: Add meta query functionality to WP_Site_Query.

After the introduction of site metadata in [42836], it should be possible to query sites by that data.

Fixes #40229.

[spacedmonkey]

@flixos90 I believe you need to put some clean_blog_cache / wp_cache_set( 'last_changed', microtime(), 'sites' ); calls into add_site_meta, update_site_meta and delete_site_meta.

Otherwise meta queries will not be invalidated.

[flixos90]

@spacedmonkey This is already in place via #37923.

[flixos90]

Version numbers need to be adjusted to 5.1.0.

[flixos90]

In 44467:

Multisite: Update @since tags for site meta introduction.

Fixes #37923. Fixes #40229.

[desrosj]

Related dev note: ​https://make.wordpress.org/core/2019/01/28/multisite-support-for-site-metadata-in-5-1/.