Matching user meta fields with custom post meta fields - php

Question

I am trying to create a function that matches user's that have the same expertise as a custom post custom field. So if a custom author meta has an expertise of 'Ninja' and the custom post type has a custom field that also has 'Ninja', my email will go out to all those matching user's.
I have the following bit of wp_mail code and can also create user queries using get_users but cannot get the two to work as i need. Any ideas?
add_action('future_to_publish', 'send_emails_on_new_event');
add_action('new_to_publish', 'send_emails_on_new_event');
add_action('draft_to_publish', 'send_emails_on_new_event');
add_action('auto-draft_to_publish', 'send_emails_on_new_event');
function send_emails_on_new_event($post) {
global $post;
$expertise = get_field('expertise', $postid);
$emails = MATCHING USER EMAIL ADDRESSES TO GO HERE;
$message = '<p>Email content will go here ...</p>';
if (get_post_type($post->ID) === 'custom_post_type') {
wp_mail($emails, "Email title will go here ...", $message);
}
}

Answer 1

Have you tried using SQL query actually to get matching emails from database??
something like
$sql = SELECT user.email FROM user WHERE user.expertise = $postExpertise;
$email = database->getRecords($sql); //getRecords() is just general function placeholder, build your own db processor class
...
function send_emails_on_new_event($post) {
...
}
You have to understand, WP is just a tool written in PHP with common functionality, if you want custom functionality, either build it yourself or if you are lucky, find plugin already created with similiar functionality. But you will not always find the plugin you want.
Hence why WP is good only for low-budget websites built by not so great web developers. Because if you can build custom functionality on top of WP, you should already consider if using WP is even good idea in first place