filter assignments in addition to submissions for DueDateCacher

helps postgres avoid a temp table

Change-Id: I33ffd1f3d35523eb350a8adff1a54eff799c95da
Reviewed-on: https://gerrit.instructure.com/30297
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

lib/due_date_cacher.rb

@@ -77,7 +77,7 @@ class DueDateCacher
           "submissions.assignment_id",
           "assignments.due_at",
           "CAST(#{Submission.sanitize(false)} AS #{cast}) AS overridden"
-        ]).joins(:assignment).to_sql})")
+        ]).joins(:assignment).where(assignments: { id: @assignments }).to_sql})")
 
         # create an ActiveRecord class around that temp table for the update_all
         scope = Class.new(ActiveRecord::Base) do