fix discussion topic materialized view for sharding

found by rails 3 specs

Change-Id: I32a8c4a63eb335fbccef05e2aac9988444b8acb7
Reviewed-on: https://gerrit.instructure.com/30836
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>