Add post meta fields, when creating a post using WordPress' REST API

Question

In the documentation for /posts for WordPress REST API it states that I can add a meta-field. But it doesn't say which format that should be in.

I can find other guides showing how it should be if using Postman:

data = {
  "title": "test",
  "content": "testingfrompython",
  "status": "draft",
  "author": 1,
  "meta": {
    "location": "NYC",
    "date": "never",
    "event_url": "http: //google.com"
  },
  "featured_media": 1221
}

But how should it be setup, if I'm call the endpoint using PHP and cURL?

---

This here works for just creating a post:

$data = [
  'title'   => 'Test post',
  'status'  => 'draft', 
  'content' => 'Some content',
];
$data_string = json_encode( $data );

$endpoint = 'https://example.org/wp-json/wp/v2/posts';
$protocol = "POST";

$headers = [
  'Content-Type: application/json',
  'Content-Length: ' . strlen($data_string)
  'Authorization: Basic ' . base64_encode( 'MYUSERSEMAIL:APPLICATIONPASSWORD' )
];

$ch = custom_setup_curl_function( $endpoint, $protocol, $data_string, $headers );
$result = json_decode( curl_exec($ch) );

I've tried all kind of things:

Attempt1

$data = [
  'title'   => 'Test post',
  'status'  => 'draft', 
  'content' => 'Some content',
  'meta' => [
    'my_custom_field' => 'Testing 1234'
  ]
];

Attempt2

$data = [
  'title'   => 'Test post',
  'status'  => 'draft', 
  'content' => 'Some content',
  'meta' => [
    [
      'key' => 'my_custom_field', 
      'value' => 'Testing 1234' 
    ]
  ]
];

... And I could keep going. Every time it simply creates a post, but doesn't add any of the meta-data to my created custom fields.

I don't get why this is not stated in the documentation for the REST API.

Answer 1

I had to add this code from Stephen Mullen on StackOverflow to get it to work:

add_action( "rest_insert_user_question", function ( \WP_Post $post, $request, $creating ) 
{
    $metas = $request->get_param( "meta" );
    if( is_array( $metas ) ) {
        foreach( $metas as $name => $value ) {
            // update_post_meta( $post->ID, $name, $value );
            update_field( $name, $value, $post->ID );
        }
    }
}, 10, 3 );

Then I could call the endpoint like this:

$data = [
  'title'   => 'Test post',
  'status'  => 'draft', 
  'content' => 'Some content',
  'meta' => [
    'my_custom_field' => 'Testing 1234', 
  ]
];

Answer 2

First, you need to register the meta key in the REST API before the REST API can handle that meta data for the post/custom post. You can do this by using the methods register_post_meta link ↗ (a convenience wrapper for register_meta link ↗ useful for custom post types) or you can use register_meta directly.

Using the register_post_meta function, you can specify the post_type to assign the field to, the meta key and an array of properties, the important property is show_in_rest.

add_action('init', function(){

    register_post_meta(
        'my_custom_post_type',
        '_custom_meta_key',
        array(
            'single'       => true,
            'type'         => 'string',
            'show_in_rest' => true,
        )
    );
});