fix cache key for discussion group assignments, fixes #5609
fix cache key for discussion group assignments, fixes #5609

Change-Id: I4e406e356e03d553b1707a73d0993c41693e33f8
Reviewed-on: https://gerrit.instructure.com/5740
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>
Reviewed-by: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

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