Topic can have null user_id when user was nuked

app/models/user_action.rb

@@ -196,10 +196,13 @@ ORDER BY p.created_at desc
           group_ids = topic.category.groups.pluck("groups.id")
         end
 
-        MessageBus.publish("/users/#{action.user.username.downcase}",
-                              action.id,
-                              user_ids: [user_id],
-                              group_ids: group_ids )
+        if action.user
+          MessageBus.publish("/users/#{action.user.username.downcase}",
+                                action.id,
+                                user_ids: [user_id],
+                                group_ids: group_ids )
+        end
+
         action
 
       rescue ActiveRecord::RecordNotUnique

db/migrate/20130904181208_allow_null_user_id_on_topics.rb

@@ -0,0 +1,9 @@
+class AllowNullUserIdOnTopics < ActiveRecord::Migration
+  def up
+    change_column :topics, :user_id, :integer, null: true
+  end
+
+  def down
+    change_column :topics, :user_id, :integer, null: false
+  end
+end

lib/user_destroyer.rb

@@ -25,6 +25,9 @@ class UserDestroyer
             end
           end
           PostDestroyer.new(@staff, post).destroy
+          if post.topic and post.post_number == 1
+            Topic.unscoped.where(id: post.topic.id).update_all(user_id: nil)
+          end
         end
         raise PostsExistError if user.reload.post_count != 0
       end

spec/components/user_destroyer_spec.rb

@@ -87,7 +87,10 @@ describe UserDestroyer do
     end
 
     context 'user has posts' do
-      let!(:post) { Fabricate(:post, user: @user) }
+      let!(:topic_starter) { Fabricate(:user) }
+      let!(:topic) { Fabricate(:topic, user: topic_starter) }
+      let!(:first_post) { Fabricate(:post, user: topic_starter, topic: topic) }
+      let!(:post) { Fabricate(:post, user: @user, topic: topic) }
 
       context "delete_posts is false" do
         subject(:destroy) { UserDestroyer.new(@admin).destroy(@user) }
@@ -123,6 +126,20 @@ describe UserDestroyer do
           post.reload.deleted_at.should_not be_nil
           post.user_id.should be_nil
         end
+
+        it "does not delete topics started by others in which the user has replies" do
+          destroy
+          topic.reload.deleted_at.should be_nil
+          topic.user_id.should_not be_nil
+        end
+
+        it "deletes topics started by the deleted user" do
+          spammer_topic = Fabricate(:topic, user: @user)
+          spammer_post = Fabricate(:post, user: @user, topic: spammer_topic)
+          destroy
+          spammer_topic.reload.deleted_at.should_not be_nil
+          spammer_topic.user_id.should be_nil
+        end
       end
     end