remove invalid topic_users rows

db/migrate/20160514100852_remove_invalid_topic_user.rb

@@ -0,0 +1,14 @@
+class RemoveInvalidTopicUser < ActiveRecord::Migration
+  def up
+    execute <<-SQL
+      DELETE FROM topic_users
+       USING topic_users tu
+   LEFT JOIN users u ON u.id = tu.user_id
+       WHERE u.id IS NULL
+         AND topic_users.id = tu.id
+    SQL
+  end
+
+  def down
+  end
+end