wp_get_attachment_metadata returns false with add_action() 'add_attachment' hook

Question

With the following code in functions.php - wp_get_attachment_metadata() returns false and or empty. Perhaps I am attempting to use wp_get_attachment_metadata() before it's ready? As some tests have shown when manually add a previously added $attachement_ID I get the desired information..

function my_custom_attachment($attachment_ID) {

  $meta = wp_get_attachment_metadata($attachment_id); // returns false

}

add_action('add_attachment', 'my_custom_attachment');

Answer 1

add_attachment runs right after an attachment is added to the DB, but but before the metadata is generated. If you don't need the value to be in the database yet, you can use the wp_generate_attachment_metadata filter to hook in. However, if you need the data to be in the database already, you'll have to use updated_post_meta and check that the meta_key passed in equals _wp_attachment_metadata.