Fix queries for counts in course data export (#501)

csm_web/scheduler/admin.py

@@ -715,7 +715,9 @@ def get_fields(self, request, obj=None):
     @admin.display(description="Section count")
     def get_section_count(self, obj: Course):
         """Retrieve the number of sections associated with this course."""
-        return obj.mentor_set.count()  # one-to-one between mentor objects and sections
+        # only count mentors that have a section associated with it;
+        # at most one section can be associated with a given mentor object
+        return obj.mentor_set.filter(~Q(section=None)).count()
 
     @admin.display(description="Student count")
     def get_student_count(self, obj: Course):

csm_web/scheduler/views/export.py

@@ -365,18 +365,20 @@ def prepare_course_data(
         export_headers.append("Course title")
     if "num_sections" in fields:
         course_queryset = course_queryset.annotate(
-            num_sections=Count("mentor__section")
+            num_sections=Count("mentor__section", distinct=True)
         )
         export_fields.append("num_sections")
         export_headers.append("Number of sections")
     if "num_students" in fields:
         course_queryset = course_queryset.annotate(
-            num_students=Count("mentor__section__students")
+            num_students=Count("mentor__section__students", distinct=True)
         )
         export_fields.append("num_students")
         export_headers.append("Number of students")
     if "num_mentors" in fields:
-        course_queryset = course_queryset.annotate(num_mentors=Count("mentor"))
+        course_queryset = course_queryset.annotate(
+            num_mentors=Count("mentor", distinct=True)
+        )
         export_fields.append("num_mentors")
         export_headers.append("Number of mentors")