REST API meta type not working properly for custom post types

[chriseverson]

When attempting to set meta via a custom post types api endpoint, the meta type is always set to post. This results in all meta updates for custom post types via the REST API failing.

For example:

$args = [
        'type'         => 'string',
        'single'       => true,
        'show_in_rest' => true
];

register_meta( 'book', 'author', $args );

If I'm understanding correctly, the above should expose the 'author' meta to the REST API. However, in the REST API, all of these fail due to a check against WP_REST_Post_Meta_Fields->get_registered_fields().

WP_REST_Post_Meta_Fields->get_registered_fields() in turn matches meta keys to get_registered_meta_keys( $this->get_meta_type() ) which always returns post.

Not sure if any of that makes sense...

The fix would be to make WP_REST_Post_Meta_Fields->get_meta_type() return the post type as far as I can tell.

I would love to tackle this one, however I wanted to get some input from someone more well-versed in the REST API codebase before tackling this to be sure that there isn't a reason that WP_REST_Post_Meta_Fields->get_meta_type() is explicitly set to return post.

[rheinardkorf]

@chriseverson I had this problem a couple of days ago. The patch will not resolve the issue.

In the docs (here: ​https://developer.wordpress.org/rest-api/extending-the-rest-api/modifying-responses/#working-with-meta-and-register_meta) you will see in the comments that for custom post types the $object_type has to be "post".

What was not documented until recently is that for meta to work is that you need to enable custom-fields support for your post type. This can be done by adding it to the "supports" array of your post type or by using add_theme_support(). See this in the docs:

Note that for meta fields registered on custom post types, the post type must have custom-fields support.

It had me tearing my hair out, but the answer ended up being simple.

[kadamwhite]

@chriseverson Thank you for opening this ticket, and thank you @rheinardkorf for clarifying that this is a deficiency of register_meta. However, the issue that @chriseverson reported is separate from the custom-fields issue; it is true that registered meta cannot be restricted to a particular custom post type, which is a major weakness of the API's current meta capabilities.

I'm going to close this as a duplicate of #38323, which is the ticket tracking the effort to provide a new argument to register_meta indicating that a particular meta value is intended only for use with a specific custom post type. I would welcome your participation in that thread!

(And if I misunderstood your issue, please note as much below and we can continue the conversation.)

[chriseverson]

Replying to rheinardkorf:

In the docs (here: ​https://developer.wordpress.org/rest-api/extending-the-rest-api/modifying-responses/#working-with-meta-and-register_meta) you will see in the comments that for custom post types the $object_type has to be "post".

Looking back through the documentation for register_meta(), I see that I incorrectly passed the post type instead of the object type, this is where I ran into issues. That said, it doesn't seem to be exactly the functionality that I'm looking for as I'd like the meta fields to be post type specific.

[chriseverson]

Replying to kadamwhite:

@chriseverson Thank you for opening this ticket, and thank you @rheinardkorf for clarifying that this is a deficiency of register_meta. However, the issue that @chriseverson reported is separate from the custom-fields issue; it is true that registered meta cannot be restricted to a particular custom post type, which is a major weakness of the API's current meta capabilities.

I'm going to close this as a duplicate of #38323, which is the ticket tracking the effort to provide a new argument to register_meta indicating that a particular meta value is intended only for use with a specific custom post type. I would welcome your participation in that thread!

(And if I misunderstood your issue, please note as much below and we can continue the conversation.)

Most excellent, thank you!