If a course is deleted or unpublished, the CourseEnrollment list serializer was causing a AttributeErrors when called for a learner who had been enrolled in the formerly-existing course.
Why no test? Because cache invalidation is the worst problem. tl;dr I could create a mock response for the `course_overview` property, but if I mocked up a response to the queryset filter in `CourseEnrollmentsApiListView`, I was effectively guessing that my code worked correctly and then creating a mock response that was an assertion of correctness. That would make any mocked test deceptive; it would appear to test behavior but it would actually just test that I had constructed a mock that passed the test.
I wanted to make an actual test for what would happen if a Course was deleted, so I made one character code fix, and then spent two days unsuccessfully attempting to completely clear out the ModuleStore so I could actually test what would happen in this instance.
IMO an actual verification by hand (which I performed, and it works) was the better part of valor.
in short, cache invalidation aaaaaaaaargh.
FIXES: APER-3913
77fe61c19c