Match REST API post output from custom endpoint

Question

What filters will transform a normal WP_Post object (as returned from get_posts()) to match the post objects returned from native REST API endpoints?

I have a custom endpoint which returns a WP_Post object, but rest_ensure_response just returns a simple JSON dump of the post:

function endpoint() {
    $p = get_posts(['include' => 10]);
    return rest_ensure_response($p);
}

//  wp-json/custom/v1/example
[{ "ID": 10, "post_date": "2017-01-23 16:15:13", "post_content": "<p>..." }] 

Returning the same post natively from the REST API remaps the data to something like this (keys are lowercase, dates are normalized, content is rendered):

// wp-json/wp/v2/posts/10
[{ "id": 10, "date": "2017-01-23T16:15:13", "content": {"rendered": "<p>..."}}]

How can I make the posts from my custom endpoint match those from native endpoints? Clearly the functions/filters exist, manually recreating them would be silly.

Answer 1

The prepare_item_for_response method of the WP_REST_Posts_Controller class "Prepares a single post output for response" by translating every property of the post object for output from the REST API. The method doesn't run statically, so we need to instantiate a controller in our endpoint.

Here's a simple example endpoint:

function endpoint( $request ) {
    $p = get_post( $request['id'] );
    $postController = new \WP_REST_Posts_Controller($p->post_type);
    $response = $postController->prepare_item_for_response( $p, $request );
    return rest_ensure_response( $response );
}

The resulting JSON response is identical to what is returned from the native endpoint.

Note that prepare_item_for_response only deals with singular WP_Post objects, a collection of posts will need to have this applied to each item.

It may also be possible to use WP_REST_Posts_Controller->get_item method. That wants to pull the post id from a request, but looks like it could be faked.