expire the canvas assignment cache after 15 minutes

fixes TALLY-501

test plan:
- setup rollcall and take attendance (make sure redis is set up)
- on the canvas side, delete the attendance assignment
- take attendance again and notice that it doesn't work
- wait 15 minutes
- take attendance again and notice that a new attendance assignment is
  automatically created (note though that not all attendance scores
  are automatically sent back to canvas, that needs to be dealt with in
  a future update)

Change-Id: I1f01e6bd443950e8110fbaaa275f4e890c9b65c5
Reviewed-on: https://gerrit.instructure.com/c/rollcall-attendance/+/226139
Tested-by: Service Cloud Jenkins <[email protected]>
Reviewed-by: Keith Garner <[email protected]>
QA-Review: Robin Kuss <[email protected]>
Product-Review: Simon Williams <[email protected]>

Author: simonista

app/models/attendance_assignment.rb

@@ -37,7 +37,7 @@ def fetch_or_create
       assignment = fetch || create
     end
 
-    redis.set(cache_key, assignment.to_json) if assignment
+    cache_assignment(assignment.to_json) if assignment
 
     assignment
   end
@@ -84,7 +84,7 @@ def update_if_needed(assignment:, update_cache: false)
 
     options = { omit_from_final_grade: course_config_omit_from_final_grade }
     updated_assignment = canvas.update_assignment(course_id, assignment['id'], options)
-    redis.set(cache_key, updated_assignment.to_json) if update_cache
+    cache_assignment(updated_assignment.to_json) if update_cache
     assignment
   end
 
@@ -135,7 +135,12 @@ def lock_key
   end
 
   def cache_key
-    "#{base_key}:assignment_cache"
+    "#{base_key}:assignment_cache_ex"
+  end
+
+  def cache_assignment(assign_json)
+    expiration = 15.minutes.seconds.to_i
+    redis.set(cache_key, assign_json, ex: expiration)
   end
 
   def active_section_ids

spec/extras/attendance_assignment_spec.rb

@@ -207,7 +207,7 @@
 
       allow(canvas).to receive(:update_assignment).and_return(updated_assignment)
 
-      expect(redis).to receive(:set).with(attendance_assignment.cache_key, updated_assignment.to_json)
+      expect(redis).to receive(:set).with(attendance_assignment.cache_key, updated_assignment.to_json, ex: 900)
       attendance_assignment.update_if_needed(assignment: { 'id' => '123', 'omit_from_final_grade' => false },
         update_cache: true)
     end