Drop deprecated database column upon upgrade

[leogermani]

I just stumbled upon some issues with (very) old blogs that still have the post_category column in the posts table.

After this column was removed 11 years ago in [10895] / #9435, we never handled their removals on db upgrades.

This is an issue because we have a magic getter in WP_Post class that will fill the non existent post_category property. However, if the column exists in the database, the magic getter isn't triggered and we get 0 every time. So I think we can consider this a bug.

Link to the magic getter -> ​https://github.com/WordPress/wordpress-develop/blob/master/src/wp-includes/class-wp-post.php#L311

Adds an upgrade routine to drop the post_category column from the wp_posts table.

This column was dropped 11 years ago from the schema but never handled by an upgrade routine. Its presence may conflict with the magic getter in WP_Post class that populates the contents if you try to directly access the post_category object attribute.

Trac ticket: https://core.trac.wordpress.org/ticket/51288

[leogermani]

Test Instructions:

Alter your local database and add a post_category column to the wp_posts table.

Apply this patch.

Reload admin.

• you should get a notice to Upgrade the DB
• Click Upgrade
• Check your database and verify the column was dropped.

Second test: Test if no errors are triggered if column not present.

Restore your current db version to force a new upgrade:

• wp option set db_version 51287
• Visit the dashboard again
• You should see the Upgrade notice
• Upgrade
• Make sure there were no errors or warnings

[davidbaumwald]

Tested the most recent patch with the instructions in comment #3 and it works as expected.

[hellofromTonya]

Per scrub today, this ticket needs a refresh and update to the DB version.

[leogermani]

Patch refreshed

[SergeyBiryukov]

In 49572:

Database: Clean up the post_category column removed from DB schema for new installations in WordPress 2.8.

Its presence may conflict with WP_Post::__get(), which should generally fill the non-existent post_category property, but is not triggered if the column exists in the database.

Follow-up to [10895].

Props leogermani, davidbaumwald, hellofromTonya.
Fixes #51288.

[desrosj]

Let's mention this in the miscellaneous developer note.

Merged in changeset https://core.trac.wordpress.org/changeset/49572

[daisyo]

Dev note published: ​https://make.wordpress.org/core/2020/11/20/miscellaneous-developer-focused-changes-in-wordpress-5-6/