don't use AR objects as hash keys

Change-Id: If81761036a28ab16a37bbc287dbf6363b5fbd483
Reviewed-on: https://gerrit.instructure.com/32329
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>