What is "meta_input" parameter in wp_insert_post() used for?

Question

I am inserting some post into wordpress using the function wp_insert_post().

I want to insert some custom fields on each post and reading the documentation I though the meta_info parameter was used for that, I tried something like this:

$data = array(
        'post_author' => 1,
        'post_status' => 'publish',
        'post_title' => $post->getTitle(),
        'post_content' => $post->getContent(),
        'post_category' => $post->getCategory(),
        'tags_input' => $post->getTags(),
        'meta_input' => array( "_test" => "testx1" )
);

$postID = wp_insert_post( $data );

The post gets inserted correctly and tags too. But there are no custom fields added. I know I could use add_post_meta() to add them but I still would like to know what the meta_input parameter is used for, because I did a search on the database for "testx1" after inserting the post and couldn't find any result.

Answer 1

This part of wp_insert_posts() gives it away:

  if ( ! empty( $postarr['meta_input'] ) ) {
        foreach ( $postarr['meta_input'] as $field => $value ) {
            update_post_meta( $post_ID, $field, $value );
        }
  } 

where we see how the post meta fields are updated/added with update_post_meta().

Here's the inline description for meta_input:

Array of post meta values keyed by their post meta key. Default empty.

This was added in WordPress 4.4 and here's relevant ticket #20451 for more information.

Note that using the underscore in front of the the meta key _test will hide it from the custom fields metabox in the post edit screen.

Answer 2

The way I do it is via term_id not slug and it works:

//insert Art items into database
$arr = array('item 1', 'item 2');
// $arr = array('art item 1', 'art item 2');

foreach ($arr as $a) { 
    wp_insert_post(array(
    //essentials
    //'ID'      => 1131,
    'post_author'       => 1,
    'post_title'        => $a,
    'post_type'         => 'post',
    'post_content'      => 'Something...',
    'post_status'       => 'publish',
    'post_name'         => 'post name',
    'meta_input'        => array( //(array) Array of post meta values keyed by their post meta key. Default empty.
        'city'     => '',// 'name' => $post['name']
        'country'  => ''// 'city' => $post['city']
    ),
    'tax_input'    => array(
        'category' => array(33,32), //id numbers work, slugs tend to be ignored !!!
        'post_tag' => array('one', 'two') //for tags slugs seem to work
    ),//(array) Array of taxonomy terms keyed by their taxonomy name. Default empty. Equivalent to calling wp_set_post_terms() / wp_set_object_terms()
    //'tags_input'  => array('una', 'trei'), //(array) Array of tag names, slugs, or IDs. Default empty. Equivalent to calling wp_set_post_tags().
    ), true);   
}