diff --git a/app/models/post_archive.rb b/app/models/post_archive.rb
index 736fcc5dc..6012489ca 100644
--- a/app/models/post_archive.rb
+++ b/app/models/post_archive.rb
@@ -205,8 +205,8 @@ class PostArchive < ActiveRecord::Base
   end
 
   def undo
-    added = changes[:added_tags_with_fields] - changes[:obsolete_added_tags]
-    removed = changes[:removed_tags_with_fields] - changes[:obsolete_removed_tags]
+    added = changes[:added_tags] - changes[:obsolete_added_tags]
+    removed = changes[:removed_tags] - changes[:obsolete_removed_tags]
 
     added.each do |tag|
       if tag =~ /^source:/
diff --git a/test/helpers/post_archive_test_helper.rb b/test/helpers/post_archive_test_helper.rb
index 7a0a19453..e47a6d32b 100644
--- a/test/helpers/post_archive_test_helper.rb
+++ b/test/helpers/post_archive_test_helper.rb
@@ -20,6 +20,12 @@ module PostArchiveTestHelper
         json.delete("created_at")
         json["version"] = 1 + PostArchive.where(post_id: json["post_id"]).count
         prev = PostArchive.where(post_id: json["post_id"]).order("id desc").first
+        if prev
+          json["added_tags"] = json["tags"].scan(/\S+/) - prev.tags.scan(/\S+/)
+          json["removed_tags"] = prev.tags.scan(/\S+/) - json["tags"].scan(/\S+/)
+        else
+          json["added_tags"] = json["tags"].scan(/\S+/)
+        end
         if merge?(prev, json)
           prev.update_columns(json)
         else
diff --git a/test/unit/post_archive_test.rb b/test/unit/post_archive_test.rb
new file mode 100644
index 000000000..83e05976c
--- /dev/null
+++ b/test/unit/post_archive_test.rb
@@ -0,0 +1,103 @@
+require 'test_helper'
+
+class PostArchiveTest < ActiveSupport::TestCase
+  context "A post" do
+    setup do
+      Timecop.travel(1.month.ago) do
+        @user = FactoryGirl.create(:user)
+      end
+      CurrentUser.user = @user
+      CurrentUser.ip_addr = "127.0.0.1"
+      MEMCACHE.flush_all
+    end
+
+    teardown do
+      CurrentUser.user = nil
+      CurrentUser.ip_addr = nil
+    end
+
+    context "#undo" do
+      setup do
+        PostArchive.sqs_service.stubs(:merge?).returns(false)
+        @post = FactoryGirl.create(:post, :tag_string => "1")
+        @post.update_attributes(:tag_string => "1 2")
+        @post.update_attributes(:tag_string => "2 3")
+      end
+
+      subject { @post.versions[1] }
+
+      should "undo the changes" do
+        subject.undo!
+        @post.reload
+        assert_equal("3", @post.tag_string)
+      end
+    end
+
+    context "that has multiple versions: " do
+      setup do
+        PostArchive.sqs_service.stubs(:merge?).returns(false)
+        @post = FactoryGirl.create(:post, :tag_string => "1")
+        @post.update_attributes(:tag_string => "1 2")
+        @post.update_attributes(:tag_string => "2 3")
+      end
+
+      context "a version record" do
+        setup do
+          @version = PostArchive.last
+        end
+
+        should "know its previous version" do
+          assert_not_nil(@version.previous)
+          assert_equal("1 2", @version.previous.tags)
+        end
+      end
+    end
+
+    context "that has been created" do
+      setup do
+        @parent = FactoryGirl.create(:post)
+        @post = FactoryGirl.create(:post, :tag_string => "aaa bbb ccc", :rating => "e", :parent => @parent, :source => "xyz")
+      end
+
+      should "also create a version" do
+        assert_equal(1, @post.versions.size)
+        @version = @post.versions.last
+        assert_equal("aaa bbb ccc", @version.tags)
+        assert_equal(@post.rating, @version.rating)
+        assert_equal(@post.parent_id, @version.parent_id)
+        assert_equal(@post.source, @version.source)
+      end
+    end
+
+    context "that should be merged" do
+      setup do
+        @parent = FactoryGirl.create(:post)
+        @post = FactoryGirl.create(:post, :tag_string => "aaa bbb ccc", :rating => "q", :source => "xyz")
+      end
+
+      should "delete the previous version" do
+        assert_equal(1, @post.versions.count)
+        @post.update_attributes(:tag_string => "bbb ccc xxx", :source => "")
+        @post.reload
+        assert_equal(1, @post.versions.count)
+      end
+    end
+
+    context "that has been updated" do
+      setup do
+        PostArchive.sqs_service.stubs(:merge?).returns(false)
+        @post = FactoryGirl.create(:post, :tag_string => "aaa bbb ccc", :rating => "q", :source => "xyz")
+        @post.update_attributes(:tag_string => "bbb ccc xxx", :source => "")
+      end
+
+      should "also create a version" do
+        assert_equal(2, @post.versions.size)
+        @version = @post.versions.last
+        assert_equal("bbb ccc xxx", @version.tags)
+        assert_equal("q", @version.rating)
+        assert_equal("", @version.source)
+        assert_nil(@version.parent_id)
+      end
+    end
+  end
+end