Framework for storing revisions of Post Meta in 6.4

RevisionsRevisions The WordPress revisions system stores a record of each saved draft or published update. The revision system allows you to see what changes were made in each revision by dragging a slider (or using the Next/Previous buttons). The display indicates what has changed in each revision. are now supported for post metaMeta Meta is a term that refers to the inside workings of a group. For us, this is the team that works on internal WordPress sites like WordCamp Central and Make WordPress. on an opt-in basis. This feature is currently used by coreCore Core is the set of software required to run WordPress. The Core Development Team builds WordPress. for footnotes – enabling footnotes to be correctly previewed, stored in autosaved and stored and retrieved from revisions. In the future, this may be expanded to the featured imageFeatured image A featured image is the main image used on your blog archive page and is pulled when the post or page is shared on social media. The image can be used to display in widget areas on your site or in a summary list of posts. which is also stored in meta.

When registering a new meta field with register_meta or register_post_meta, the $args parameter accepts a new revisions_enabled argument. This defaults to false—set to true to opt in to revisions for this meta field. Note that the object type the meta is being created for must have revision support in order for the meta field to have revisions enabled. Attempting to enable meta revisions for a post type that doesn’t support revisions will result in a doing_it_wrong warning.

This feature is designed to lay the groundwork for future improvements to GutenbergGutenberg The Gutenberg project is the new Editor Interface for WordPress. The editor improves the process and experience of creating new content, making writing rich content much simpler. It uses ‘blocks’ to add richness rather than shortcodes, custom HTML etc. as part of Phase 3: Collaboration. Care has been made to do this in a backwards compatible manner, but if you are unhooking anything in the post save/update process, please read this entire note and test your code before WordPress 6.4 is released

A new wp_post_revision_meta_keys filterFilter Filters are one of the two types of Hooks They provide a way for functions to modify data of other functions. They are the counterpart to Actions. Unlike Actions, filters are meant to work in an isolated manner, and should never have side effects such as affecting global variables and output. is introduced, which can be used to filter the list of meta fields to be revisioned. The post type is also passed to the filter, which can be used to adjust revisioning of meta fields.

// Example to add a `my_product_price` meta field to revisions.
function enable_revisions_for_product_price_field( $revisioned_keys ) {
	if ( ! in_array( 'my_product_price', $revisioned_keys, true ) ) {
		$revisioned_keys[] = 'product_price';
	return $revisioned_keys;
add_filter( 'wp_post_revision_meta_keys', 'enable_revisions_for_product_price_field' );

The _wp_put_post_revision action, which fires once a revision is saved, includes a new parameter—the post ID associated with the revision. This is used by core to copy the meta data from the main post to the revision.

The wp_creating_autosave action, which fires before an autosave is stored, includes a new parameter `$is_update`, added to indicate if the autosave is being updated or was newly created.

Meta revisions are also stored for autosaves and when previewing posts, fixing a long standing bugbug A bug is an error or unexpected result. Performance improvements, code optimization, and are considered enhancements, not defects. After feature freeze, only bugs are dealt with, with regressions (adverse changes from the previous version) being the highest priority. (#20299) that caused live post meta to be overwritten when post meta was previewed. This is because using update_post_meta to save meta to a revision actually resulted in it being saved to the parent post. With this change, developers can now opt in to meta revisions and core will correctly attach the meta data to the revision. Meta that has revisions is also restored when restoring an autosave or revision. 

The REST APIREST API The REST API is an acronym for the RESTful Application Program Interface (API) that uses HTTP requests to GET, PUT, POST and DELETE data. It is how the front end of an application (think “phone app” or “website”) can communicate with the data store (think “database” or “file system”) revisions endpoint now returns meta when stored as part of a revision and the autosaves endpoint now handles storing of meta.

The new meta revision features are enabled in core via hooksHooks In WordPress theme and development, hooks are functions that can be applied to an action or a Filter in WordPress. Actions are functions performed when a certain event occurs in WordPress. Filters allow you to modify certain functions. Arguments used to hook both filters and actions look the same.; removing the hooks will disable the features:

To disable post meta revisions entirely, use:

remove_action( 'wp_after_insert_post', 'wp_save_post_revision_on_insert', 9, 3 );

To disable storing post meta on autosaves, use:

remove_action( 'wp_creating_autosave', 'wp_autosave_post_revisioned_meta_fields' );

To disable restoring meta when restoring revisions or autosaves, use:

remove_action( 'wp_restore_post_revision', 'wp_restore_post_revision_meta', 10, 2 );

Important note: as part of this change, the timing for the storing of post revisions has been changed. Previously, revisions were created before post meta had been saved by the REST API on the post_updated hook.  Revisions are now hooked on the wp_after_insert_post action, fired after post meta has been saved. 

Special backwards compatibility consideration has been given to plugins that may be unhooking post_updated from wp_save_post_revision to disable revisions—in that case, core ensures revisions are still not created.

Props to @westonruter, @jorbin and @webcommsat for reviewing.

#6-4-2, #dev-notes, #dev-notes-6-4