Custom "Post Author" Input Instead of Dropdown - php

Question

I have a site with over 20,000 users. Occasionally I need to update the author of a post. However, the sheer amount of users makes the "Author" metabox on the edit post screen unusable. I'm trying to make a custom box where I can input the desired author's ID and change the post author to that ID. Ideally, I would like to do this within the box itself, rather than by way of saving/updating the post.
Here's my custom meta box code:
//Get the post data
$id = get_the_ID();
$author_id= $post->post_author;
<div class="update-author">
<p><strong>Current Author ID:</strong> <?php echo $author_id;?></p>
<p><strong>CHANGE AUTHOR ID TO:</strong></p>
<form id="update-author-form" method="post" action="<?php echo esc_url( admin_url('admin-post.php') ); ?>" >
<input type="hidden" name="action" value="update_author_form">
<input type="hidden" name="post-id" value="<?php echo $id; ?>">
<input type="number" id="update-author-id" name="update-author-id" value="" />
<button id="update-author-button" class="button button-primary button-large">Update Author ID</button>
</form>
</div>
Next, I have my form function built as follows:
function handle_update_author_form() {
$post_id = $_POST['post-id'];
$update_id = $_POST['update-author-id'];
$my_post = array(
'ID' => $post_id,
'post_author' => $update)_id,
);
wp_update_post( $my_post );
}
At this stage, my form button does nothing but refresh the page and send it to the posts screen (I know, I need to change the redirect URL).

Answer 1

Like in comments - I would go for a another approach of removing the original meta_box and replacing it with a new one - or alternatively just use a JS to turn the dropdown list into a searchable field ( for example the excellent select2 plugins - but have others - and also pure JS is quite easy - see example below )
Anyhow - in your code that you posted you are missing the hook where you can apply and invoke your code, for example:
'add_action( 'save_post', 'handle_update_author_form', 10, 1 ); '
Have a look at wp filters and actions.
The suggested (simple) JS solution :
Now, for the simple approach of just using JS to pseudo-search inside the dropdown and therefor there is no real need to interfere with the query or post injections.....
myPlugin.php / myTHeme,php
function admin_enqueue_scripts_callback(){
// We are using select2.js from CDN here ..
//This will add Select2 CSS
wp_enqueue_style( 'select2-css', 'https://cdn.jsdelivr.net/npm/select2#4.1.0-rc.0/dist/css/select2.min.css', array(), '4.1.0-rc.0');
////This will add Select2 JS
wp_enqueue_script( 'select2-js', 'https://cdn.jsdelivr.net/npm/select2#4.1.0-rc.0/dist/js/select2.min.js', 'jquery', '4.1.0-rc.0');
// Now we need a JavaScript file to initialize the Select2 elements
// here it is staged as a plugin - but a theme would be the same .
wp_enqueue_script( 'select2-init', '/wp-content/plugins/select2-init.js', 'jquery', '4.1.0-rc.0');
}
add_action( 'admin_enqueue_scripts', 'admin_enqueue_scripts_callback' );
select2-init.js
// initialize select2 on author dropdown field with jQuery
jQuery(document).ready(function($) {
$('#post_author_override').select2();
});
voilà !
At this point you will have the pseudo-search implemented that would resolve your problem in a practical way without any new ->queries or $objects and with minimum coding.
Possible / potential caveats
I am not sure what would be the efficiency of filtering 20,000 items.
It would be interesting to know how the select2 JS would perform with such a long list...
But if it is already loaded to the DOM i guess it would be ok ( might take a few seconds to react though )
Please let me know how it performing if you implement the solution.