How to access post meta on the first time a post is published

Question

Am trying to send an email based on the contents of a post published but it should only be done if its the first time the post is published.

Therefore am using add_action( 'transition_post_status', [ $this, 'jackie_publish_post_hook'], 20, 3 );

the callback function jackie_publish_post_hook has the following content.(The full content is quite big and some information is irrelevant to this question therefore i will only include the relative part)

public function jackie_publish_post_hook($new,$old,$post){

...

$accepted = array('pending','future','draft','auto-draft');
if(in_array($old, $accepted) && $new == 'publish'){
    $attachment = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'full', true);
    print_r($attachment);
}

My problem occurs when I try to access the any post meta eg the featured image or any custom field as they have not been saved yet to database.

Some solutions have suggested using add_action( 'publish_post', ... ); and then accessing the values from the $_POST variable but when I do this the $_POST returns an empty array.
I have confirmed from the developers tools that the featured image id is being sent as a post parameter with the request but I cannot seem to access it from the given callback.

If I force the function to run after a post has been published earlier it returns the correct data. ie running manually. $this->jackie_publish_post_hook('publish','draft',get_post(70801));

Is there a special way to access the first time submitted data from the hook callback function or what am I doing wrong?

Can I access the post meta from unsaved but submitted data to wordpress?

Answer 1

I usually handle this with save_post action.

// We'll use 1000, low priority (we'll let other hooks to run first)
add_action( 'save_post', [$this, 'jackie_publish_post_hook'], 1000, 3 ); 

Then within your class:

function jackie_publish_post_hook( $post_id, $post, $update ) {
    if ( $update ) return false; // Post already existed, don't proceed

    // Do stuff
}

Or use a post meta:

function jackie_publish_post_hook( $post_id, $post, $update ) {
    if ( get_post_meta( $post_id, 'jackie_post_processed', true ) != '' ) return false;
    update_post_meta( $post_id, 'jackie_post_processed', 1 );

    // Do stuff
}