don't overwrite @shard on initialize

Change-Id: I958696cfcefdf22c5aee5a29704e20f8957fd2b2
Reviewed-on: https://gerrit.instructure.com/28985
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
fetch singular associations on shard implied by foreign key

Change-Id: I7fbe0e514dae7e501bd682019156b516d97a8a5c
Reviewed-on: https://gerrit.instructure.com/28993
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>