Return distinct list of available courses.

The query that fetches available courses performs a join across the
course table, the course_runs table, and the seats table and returns
a separate copy of the same course for each available run/seat
combination by default.

We have to use distinct() to make sure only a single copy of each
available course is returned.

course_discovery/apps/course_metadata/query.py

@@ -50,7 +50,14 @@ class CourseQuerySet(models.QuerySet):
         # runs is published while the other is not. If you used exclude(), the Course
         # would be dropped from the queryset even though it has one run which matches
         # our availability criteria.
-        return self.filter(enrollable & not_ended & marketable)
+        query = self.filter(enrollable & not_ended & marketable)
+
+        # By itself, the query above performs a join across several tables and would return
+        # a copy of the same course multiple times (a separate copy for each available
+        # seat in each available run).
+        # We use distinct() to make sure it only returns a single copy of each available
+        # course.
+        return query.distinct()
 
 
 class CourseRunQuerySet(models.QuerySet):

course_discovery/apps/course_metadata/tests/test_query.py

@@ -30,14 +30,24 @@ class CourseQuerySetTests(TestCase):
 
             if state in self.available_states:
                 course = course_run.course
+                # This course is available, so should be returned by the
+                # available() query.
+                assert list(Course.objects.available()) == [course]
 
                 # This run has no seats, but we still expect its parent course
                 # to be included.
                 CourseRunFactory(course=course)
+                assert list(Course.objects.available()) == [course]
 
-                assert set(Course.objects.available()) == {course}
+                # Generate another course run with available seats.
+                # Only one instance of the course should be included in the result.
+                other_course_run = CourseRunFactory(course=course)
+                for function in state:
+                    function(other_course_run)
+                other_course_run.save()
+                assert list(Course.objects.available()) == [course]
             else:
-                assert set(Course.objects.available()) == set()
+                assert list(Course.objects.available()) == []
 
 
 @ddt.ddt