Fix #3480: NoMethodError error when adding artist tag to post.

db/migrate/20171230220225_change_timestamps_to_non_null_on_tags.rb

@@ -0,0 +1,8 @@
+class ChangeTimestampsToNonNullOnTags < ActiveRecord::Migration
+  def change
+    Post.without_timeout do
+      change_column_null :tags, :created_at, false
+      change_column_null :tags, :updated_at, false
+    end
+  end
+end