fix #1724 for posts

app/models/post_version.rb

@@ -112,4 +112,31 @@ class PostVersion < ActiveRecord::Base
   def truncated_source
     source.gsub(/^http:\/\//, "").sub(/\/.+/, "")
   end
+
+  def undo
+    changes = diff(previous)
+    added = changes[:added_tags] - changes[:obsolete_added_tags]
+    removed = changes[:removed_tags] - changes[:obsolete_removed_tags]
+
+    added.each do |tag|
+      if tag =~ /^source:/
+        post.source = ""
+      else
+        escaped_tag = Regexp.escape(tag)
+        post.tag_string = post.tag_string.sub(/(?:\A| )#{escaped_tag}(?:\Z| )/, " ").strip
+      end
+    end
+    removed.each do |tag|
+      if tag =~ /^source:(.+)$/
+        post.source = $1
+      else
+        post.tag_string = "#{post.tag_string} #{tag}".strip
+      end
+    end
+  end
+
+  def undo!
+    undo
+    post.save!
+  end
 end