Avoid subquery on table being updated

MySQL doesn't like you subquerying on the same table you are trying
to update in one query.

https://dev.mysql.com/doc/refman/8.0/en/subquery-restrictions.html

PROD-1366

openedx/core/djangoapps/schedules/tests/test_signals.py

@@ -242,29 +242,33 @@ class ResetScheduleTests(SharedModuleStoreTestCase):
         self.enrollment = CourseEnrollmentFactory(
             course_id=self.course.id,
             mode=CourseMode.AUDIT,
+            is_active=False,
         )
         self.schedule = self.enrollment.schedule
+        self.user = self.enrollment.user
 
     def test_schedule_is_reset_after_enrollment_change(self):
         """ Test that an update in enrollment causes a schedule reset. """
         original_start = self.schedule.start_date
 
-        CourseEnrollment.enroll(self.enrollment.user, self.course.id, mode=CourseMode.VERIFIED)
+        CourseEnrollment.enroll(self.user, self.course.id, mode=CourseMode.VERIFIED)
 
         self.schedule.refresh_from_db()
         self.assertGreater(self.schedule.start_date, original_start)  # should have been reset to current time
 
-    def test_schedule_is_reset_to_availabilty_date(self):
-        """ Test that a switch to audit enrollment resets to the availabilty date, not current time. """
+    def test_schedule_is_reset_to_availability_date(self):
+        """ Test that a switch to audit enrollment resets to the availability date, not current time. """
         original_start = self.schedule.start_date
 
         # Switch to verified, confirm we change start date
-        CourseEnrollment.enroll(self.enrollment.user, self.course.id, mode=CourseMode.VERIFIED)
+        CourseEnrollment.enroll(self.user, self.course.id, mode=CourseMode.VERIFIED)
         self.schedule.refresh_from_db()
         self.assertNotEqual(self.schedule.start_date, original_start)
 
+        CourseEnrollment.unenroll(self.user, self.course.id)
+
         # Switch back to audit, confirm we change back to original availability date
-        CourseEnrollment.enroll(self.enrollment.user, self.course.id, mode=CourseMode.AUDIT)
+        CourseEnrollment.enroll(self.user, self.course.id, mode=CourseMode.AUDIT)
         self.schedule.refresh_from_db()
         self.assertEqual(self.schedule.start_date, original_start)
 

openedx/core/djangoapps/schedules/utils.py

@@ -8,6 +8,7 @@ from django.db.models import F, Subquery
 from django.db.models.functions import Greatest
 
 from openedx.core.djangoapps.schedules.models import Schedule
+from student.models import CourseEnrollment
 
 LOG = logging.getLogger(__name__)
 
@@ -54,7 +55,18 @@ def reset_self_paced_schedule(user, course_key, use_availability_date=False):
     )
 
     if use_availability_date:
-        schedule = schedule.annotate(start_of_access=Greatest(F('enrollment__created'), F('enrollment__course__start')))
-        schedule.update(start_date=Subquery(schedule.values('start_of_access')[:1]))
+        # Query enrollments to find availability date -- very similar to query above, but we can't reuse that query
+        # object because mysql doesn't like a subquery of an update to reference the same table being updated.
+        # Be careful attempting to remove this logic because you can't reproduce a problem locally -- in my own testing,
+        # I could not reproduce in devstack, but it was happening on prod databases. So implementations vary.
+        # See https://dev.mysql.com/doc/refman/8.0/en/subquery-restrictions.html
+        enrollments = CourseEnrollment.objects.filter(
+            user=user,
+            course__id=course_key,
+            course__self_paced=True,
+        ).annotate(
+            availability=Greatest(F('created'), F('course__start')),
+        )
+        schedule.update(start_date=Subquery(enrollments.values('availability')[:1]))
     else:
         schedule.update(start_date=datetime.datetime.now(pytz.utc))