How To Display Tags From Certain Categories

Just spent about 2-3 hours trying to figure this out, so I thought I would post it to save time for the next unfortunate person to have to do it.

My problem was that I had a blog page and wanted to display all the tags for the posts on that page in the sidebar, but I only wanted to list tags from the category that the blog posts were in (‘News’ in my case). After way too much searching and trying a plugin that broke my site, I came up with this nice, compact solution:

$tags = new WP_Query('showposts=999&cat=8');
			while ($tags->have_posts()) : $tags->the_post();
			foreach ((wp_get_post_tags(get_the_ID(), array('fields' => 'ids'))) as $tagid) {
				$whichtags .= $tagid . ',';
			}
			endwhile;

			echo "<div class='tagcloud'><h1>tags</h1>";
			wp_tag_cloud("include=" . $whichtags);
			echo "</div>";

Simply change the ‘cat’ argument in the $tags query to whatever category ID you need.

Hope this saves some time for others who run into this issue.