WP_Posts_List_Table cannot order by meta value correctly

[intoxstudio]

Sorting by meta_value in tables that extend the WP_Posts_List_Table class does no longer work as expected. Previously it would be done by adding the column in the manage_edit-{$post_type}_sortable_columns filter and then injecting something like following into the posts query:

array(
	'meta_key' => 'some_key',
	'orderby'  => 'meta_value'
)

Then, when you click on a column header, ascending and descending sorting would be handled automatically.

Not anymore. If you click on a column header with sorting by meta_value, it only works ascending (or descending if this is default).

This is caused by this function in WP_Posts_List_Table:

protected function get_orderby() {
	return strtolower( get_query_var( 'orderby' ) );
}

It will return "meta_value" when sorting by a meta value, and thus the column will never be recognized when rendering the sortable header (unless its name is in fact meta_value).

Before WP4.4, the check would be on $_GET["orderby"] that would return the actual sorting key, in this example "some_key".

I like the idea of the new abstraction of get_orderby(), but why was it changed from $_GET? Would anything break if we changed it back?

E.g.:

protected function get_orderby() {
	return strtolower( isset($_GET["orderby"]) ? $_GET["orderby"] : "" );
}

Probably related: #34479

[boonebgorges]

get_orderby() was introduced in [34728]. See #25493.

This appears to be the same issue as what's described in https://core.trac.wordpress.org/ticket/25493#comment:9, so I'm going to mark this ticket as a duplicate. @intoxstudio - please follow along on that ticket, and thanks for the report.