WordPress provides a powerful REST API that allows developers to extend the functionality of their custom post types (CPTs) and retrieve data in various formats. However, exposing custom fields through the REST API requires careful implementation. In this guide, we’ll walk through a step-by-step process to troubleshoot and resolve issues when custom fields are not being exposed as expected.

1. Initial Code Examination:

The provided code attempts to expose a custom field named video for the project custom post type. The register_featured_video function is hooked into the rest_api_init action, attempting to register the custom field. The get_featured_video function retrieves the custom field value.

2. Check CPT Registration:

Ensure that the custom post type project is registered with the ‘show_in_rest’ parameter set to true. This can be done when registering the post type using the register_post_type function.

register_post_type(‘project’, array(
    ‘show_in_rest’ => true,
    // other parameters…

3. Debugging Techniques:

a. Enable Debugging: Check for errors by enabling WordPress debugging. Add the following lines to your wp-config.php file.

define(‘WP_DEBUG’, true);
define(‘WP_DEBUG_DISPLAY’, false);
define(‘WP_DEBUG_LOG’, true);

b. Check Error Log: If you’re not seeing the debug log file, use error_log() within your functions to print debugging information. For example:

error_log(‘Your debug message here’);

4. Registering Custom Fields:

Try using the register_post_meta function directly to register the custom field within the functions.php file.

register_post_meta(‘project’, ‘video’, array(‘show_in_rest’ => true));

5. Controller and Route Modification:

The issue might stem from how the REST route and callback are registered. Consider extending the default controller class and modifying the callback to return additional fields.

class WP_REST_Projects_Controller extends WP_REST_Posts_Controller {
    // … your custom code here …
add_action(‘rest_api_init’, function () {
    (new WP_REST_Projects_Controller(‘project’))->register_routes();

add_action(‘rest_api_init’, function () {

(new WP_REST_Projects_Controller(‘project’))->register_routes();

6. Debugging Single Project Endpoint:

The single_project callback should return only the requested post, not all posts. Ensure that the callback is correctly processing the requested post.

7. Integrating Controllers:

If using custom endpoints, explicitly define the schema and data handling. If extending default controllers, take advantage of filters and piggyback on WordPress functionality.

8. Common Pitfalls:

a. Post Type Support: Confirm that the custom post type supports custom fields in its registration.

b. Schema Definition: Explicitly define the schema and data handling for custom endpoints.


Exposing custom fields through the WordPress REST API may involve various considerations, and the provided code might need adjustments based on your specific use case. By following these steps and ensuring proper registration, debugging, and route handling, you can successfully expose custom fields for your custom post types in the WordPress REST API. If issues persist, consider reaching out to the WordPress community for additional support.

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