Sometimes, an array is passed to the get_callback provided to register_rest_field() instead of an object

[salzano]

When using register_rest_field() to add fields to terms in the WordPress REST API, the $object sent to your get_callback function will be an array instead of an object like the documentation describes.

An object will still be passed, sometimes, too. Here is an example of how I work around this problem:

	function add_api_term_fields() {

		//location-phone-hours
		register_rest_field( 'location', 'location-phone-hours', array(
			'get_callback'    => array( $this, 'get_term_meta_via_rest' ),
			'update_callback' => array( $this, 'set_term_meta_via_rest' ),
			'schema'          => array(
				'description' => __( 'An array of phone numbers and hours of operation for this location.', 'inventory-portal' ),
				'type'        => 'string',
				'context'     => array( 'view', 'edit' ),
			),
		) );
	}

	static function get_term_meta_via_rest( $term, $attr, $request, $object_type ) {
		$term_id = 0;

		if(  is_array( $term ) ) {
			$term_id = $term['id']
		} else {
			$term_id = $term->term_id;
		}

		return maybe_serialize( get_term_meta( $term_id, $attr, true ) );
	}

Why is this happening? See the definition of prepare_item_for_response() method on line 683 of wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php. The call to add_additional_fields_to_object() on line 725 is the method that passes an array to your get_callback.

Perhaps this is an acceptable design. I believe the only reference to the data type of $object in the documentation is this comment ​https://developer.wordpress.org/reference/functions/register_rest_field/#comment-2260. It certainly feels like a bug.

[TimothyBlynJacobs]

Assigning this to myself to investigate. I think at this point we can really only hope for a docs fix to clarify without breaking BC.

[TimothyBlynJacobs]

I looked at all the usages of add_additional_fields_to_object and update_additional_fields_for_object

Autosaves
- Add Fields: Passes prepared data as an array.

Comments
- Add Fields: Passes prepared data as an array.
- Update fields: Passes WP_Comment object.

Post Statuses
- Add Fields: Passes prepared data as an array.

Post Types
- Add Fields: Passes prepared data as an array.

Posts
- Add Fields: Passes prepared data as an array.
- Update fields: Passes WP_Post object.

Revisions
- Add Fields: Passes prepared data as an array.

Search
- Add Fields: Passes prepared data as an array.

Taxonomies
- Add Fields: Passes prepared data as an array.

Terms
- Add Fields: Passes prepared data as an array.
- Update fields: Passes WP_Term object.

Themes
- Add Fields: Passes prepared data as an array.

Users
- Add Fields: Passes prepared data as an array.
- Update fields: Passes WP_User object.

Attachments
- Update fields: Passes WP_Post object.

As far as I can tell, the REST API always passes an array to the get_callback() and always passes an entity object to the update_callback.

An object will still be passed, sometimes, too.

Could you clarify when this happens?

Perhaps this has been fixed in the meantime?

The documentation certainly could use updating as well.

[kadamwhite]

In 45810:

REST API: Clarify arguments passed to rest route get & update callbacks.

Update doc block argument definitions to clarify that the REST API always passes an array to the get_callback and always passes an entity object to the update_callback.

Props TimothyBlynJacobs, salzano.
Fixes #44432.

[kadamwhite]

Committed the docs change clarifying this behavior for 5.3.

[kadamwhite]

@salzano Thank you for calling this out. It does appear to my eyes that the functionality is internally consistent and that the major oversight was a lack of documentation. I've committed @TimothyBlynJacobs' patch to update the documentation, and provisionally closed this as fixed; if however you determine that there is a case we're overlooking please note it here or message us to indicate this should be reopened and continued. Thank you for your contribution!