Custom WordPress Metabox: Relation between two CPT's

functions.php

<?php
/*
To create a relationship metabox between two custom post types in WordPress, you'll need to use the add_meta_box() function 
and the WP_Query class. Here are the steps you can follow:

Create a custom post type using the register_post_type() function for both post types.

In the functions.php file of your theme or plugin, add the following code to create a metabox:
*/

function my_relationship_metabox() {
    add_meta_box(
        'my-relationship-metabox',
        'Related Posts',
        'my_relationship_metabox_callback',
        array('post_type_1', 'post_type_2'),
        'side',
        'default'
    );
}
add_action( 'add_meta_boxes', 'my_relationship_metabox' );
/*
This code will create a metabox titled "Related Posts" on the edit screen of both custom post types.

Next, define the callback function my_relationship_metabox_callback that will generate the content of the metabox. 
In this function, you can use a WP_Query object to get the related posts. Here is an example of how to do this:
*/
function my_relationship_metabox_callback( $post ) {
    $args = array(
        'post_type' => 'post_type_1',
        'meta_query' => array(
            array(
                'key'     => '_related_post_id',
                'value'   => $post->ID,
                'compare' => '=',
            ),
        ),
    );
 
    $query = new WP_Query( $args );
 
    // Display the related posts
}
/*
In this example, we're querying all posts of post type 1 that have a custom field _related_post_id matching the current post's ID.

Finally, you'll need to add some JavaScript to handle the selection of related posts. 
You can use the WordPress built-in wp_enqueue_script() function to register and enqueue your custom script.
*/
function my_admin_scripts() {
    wp_enqueue_script( 'my-admin-js', plugin_dir_url( __FILE__ ) . 'js/my-admin.js', array( 'jquery' ), '1.0', true );
}
add_action( 'admin_enqueue_scripts', 'my_admin_scripts' );
In this example, we're registering and enqueuing a script called my-admin.js.

In the my-admin.js file, you can use jQuery to add an autocomplete input field that will allow users to search for related posts. Here is an example of how to do this:
jQuery(document).ready(function($) {
    $('#related_posts').autocomplete({
        source: ajaxurl + '?action=my_relationship_metabox_search',
        minLength: 2,
        select: function(event, ui) {
            event.preventDefault();
            // Add the selected post to the related posts list
        }
    });
});
/*
In this example, we're using the autocomplete() function from the jQuery UI library. We're also specifying an AJAX URL that will 
trigger a server-side function to search for related posts based on user input.

Finally, you'll need to define the my_relationship_metabox_search function that will handle the AJAX request and return the 
search results. Here's an example of how to do this:
*/
function my_relationship_metabox_search() {
    $term = $_GET['term'];
 
    $args = array(
        'post_type' => 'post_type_1',
        's' => $term,
    );
 
    $query = new WP_Query( $args );
 
    $results = array();
 
    foreach ( $query->posts as $post ) {
        $results[] = array(
            'id' => $post->ID,
            'label' => $post->post_title,
            'value' => $post->post_title,
        );
    }
 
    wp_send_json( $results );
}
add_action( 'wp_ajax_my_relationship_metabox_search', 'my_relationship_metabox_search' );

/*
In this example, we're using WordPress's AJAX API to handle the request. We're also using the s parameter to search for posts 
that match the user's input.

That's it! With these steps, you should have a working relationship metabox between two custom post types in WordPress.
*/