User::getChatWith

public function getChatWith(int $userId): ?PrivateChat
{
	$chatId = false;
	if ($userId === $this->getId())
	{
		$result = FavoriteChat::find(['TO_USER_ID' => $userId])->getResult();
		if ($result && isset($result['ID']))
		{
			$chatId = (int)$result['ID'];
		}
	}
	else
	{
		$result = RelationTable::query()
			->setSelect(['CHAT_ID'])
			->registerRuntimeField(
				'SELF',
				new Reference(
					'SELF',
					RelationTable::class,
					Join::on('this.CHAT_ID', 'ref.CHAT_ID'),
					['join_type' => Join::TYPE_INNER]
				)
			)->where('USER_ID', $this->getId())
			->where('SELF.USER_ID', $userId)
			->where('MESSAGE_TYPE', IM_MESSAGE_PRIVATE)
			->setLimit(1)
			->fetch()
		;
		if ($result && isset($result['CHAT_ID']))
		{
			$chatId = (int)$result['CHAT_ID'];
		}
	}

	if ($chatId !== false)
	{
		$chat = PrivateChat::getInstance($chatId);

		if ($chat instanceof PrivateChat)
		{
			return $chat;
		}

		return null;
	}

	try
	{
		$createResult = (new PrivateChat())->add(['FROM_USER_ID' => $this->getId(), 'TO_USER_ID' => $userId]);
	}
	catch (SystemException $exception)
	{
		return null;
	}

	if (!$createResult->isSuccess())
	{
		return null;
	}

	return $createResult->getResult()['CHAT'];
}