How to retrieve the first message of a forum post

[AlexCai2019]

I tried using forumPost.retrieveParentMessage(), but because the parent channel of a forum post is not a MessageChannel but a ForumChannel, hence it throws an UnsupportedOperationException.
Here is part of the code that throws an exception:

private static final Emoji reminder_ribbon = Emoji.fromUnicode("🎗️");
private static final String reminder_ribbonFormat = reminder_ribbon.getFormatted();

public static void archiveForumPost(ThreadChannel forumPost)
{
	forumPost.retrieveParentMessage().queue(message ->
	{
		if (message.getReactions().stream().anyMatch(messageReaction -> messageReaction.getEmoji().getFormatted().equals(reminder_ribbonFormat)))
			message.removeReaction(reminder_ribbon, forumPost.getJDA().getSelfUser()).queue();
	});
}

[MinnDevelopment]

You can use getIterableHistory:

channel.getIterableHistory()
  .reverse() // iterate from the beginning of the channel history
  .limit(1) // only take first message
  .queue(messages -> {
    Message message = messages.get(0); // get first from list (might throw if channel is empty)
  });

[AlexCai2019]

Update: after v5.0.0-beta.10, you can use retrieveStartMessage:

channel.retrieveStartMessage().queue(message ->
{
  ///TODO: put your code here.
});

[MinnDevelopment]

Note that this is not necessarily the first message. The start message can be deleted, in which case this request fails, while a history request will just return the first message in history.