CDBT: How to display the input fields (select list, combobox) that is related the column of foreign tables by using filter hook.

table_relation_fields.php

<?php
function my_shortcode_custom_forms( $elements_options, $shortcode_name, $table ){
  if ( 'master_data' === $table ) {
    global $cdbt;
    $cities = $cdbt->get_data( 'cities' );
    $_city_list = [];
    $jobs = $cdbt->get_data( 'jobs' );
    $_job_list = [];
    foreach ( $elements_options as $_i => $_option ) {
      $_default_value = $elements_options[$_i]['defaultValue'];
      switch ( $_option['elementName'] ) {
        case 'city_id':
          $elements_options[$_i]['elementType'] = 'select';
          foreach ( $cities as $_city ) {
            $_city_list[] = $_city->city_name .':'. $_city->city_id;
            if ( intval( $_default_value ) == intval( $_city->city_id ) ) 
              $elements_options[$_i]['defaultValue'] = $_city->city_id;
          }
          $elements_options[$_i]['selectableList'] = implode( ',', $_city_list );
          break;
        case 'job':
          $elements_options[$_i]['elementLabel'] = 'Job (combobox)';
          $elements_options[$_i]['elementType'] = 'combobox';
          foreach ( $jobs as $_job ) {
            $_job_list[] = $_job->job_name .':'. $_job->job_name;
            if ( $_default_value === $_job->job_name ) 
              $elements_options[$_i]['defaultValue'] = $_job->job_name;
          }
          $elements_options[$_i]['elementSize'] = 3;
          $elements_options[$_i]['selectableList'] = implode( ',', $_job_list );
          break;
      }
    }
  }
  return $elements_options;
}
add_filter( 'cdbt_shortcode_custom_forms', 'my_shortcode_custom_forms', 10, 3 );

/**
 * Reference of structure
 * ---

CREATE TABLE `master_data` (
  `ID` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'ID',
  `name` varchar(100) DEFAULT NULL,
  `city_id` int(11) NOT NULL COMMENT 'City',
  `job` varchar(200) NOT NULL,
  `created` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Created Datetime',
  `updated` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT 'Updated Datetime',
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `cities` (
  `city_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'City ID',
  `city_name` varchar(100) NOT NULL,
  PRIMARY KEY (`city_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

| city_id | city_name |
|---------|-----------|
| 1       | Tokyo     |
| 2       | Kyoto     |
| 3       | Osaka     |


CREATE TABLE `jobs` (
  `job_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Job ID',
  `job_name` varchar(100) NOT NULL,
  PRIMARY KEY (`job_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

| job_id | job_name         |
|--------|------------------|
| 1      | Civil Servant    |
| 2      | Company Employee |
| 3      | Housewife        |

*/