Relationships between multiple databases

Problem/Motivation

It seems currently only view option is available for multiple databases and you cannot create "relationships" between:

Steps:
1.Update settings.php

$databases['default']['default'] = array (
  'database' => 'drupal_database_1',
  'username' => 'yourname',
  'password' => 'yourpass,
  'prefix' => '',
  'host' => '127.0.0.1',
  'port' => '3306',
  'namespace' => 'Drupal\\mysql\\Driver\\Database\\mysql',
  'driver' => 'mysql',
  'autoload' => 'core/modules/mysql/src/Driver/Database/mysql/',
);

$databases['second']['default'] =  [
  'database' => 'custom_database_2',
  'username' => 'yourname',
  'password' => 'yourpass',
  'prefix' => '',
  'host' => '127.0.0.1',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
];

2.Now refresh cache and you should be able to see second database as well.
3.Add table from second databse
3.Try to create a relationship - you cannot see first database there, hence unable connect custom_db with a drupal one :(