fix update group idempotence (#899)

Author: YaleChen299

lib/cadet/courses/courses.ex

@@ -242,12 +242,16 @@ defmodule Cadet.Courses do
 
         # If admin or staff, remove their previous group assignment and set them as group leader
         _ ->
-          remove_staff_from_group(course_id, course_reg.id)
-          update_course_reg_group(course_reg, group.id)
-
-          group
-          |> Group.changeset(%{leader_id: course_reg.id})
-          |> Repo.update()
+          if group.leader_id != course_reg.id do
+            update_course_reg_group(course_reg, group.id)
+            remove_staff_from_group(course_id, course_reg.id)
+
+            group
+            |> Group.changeset(%{leader_id: course_reg.id})
+            |> Repo.update()
+          else
+            {:ok, nil}
+          end
       end
     end
   end

test/cadet/courses/courses_test.exs

@@ -524,6 +524,18 @@ defmodule Cadet.CoursesTest do
 
       leader = CourseRegistration |> Repo.get(new_group_leader.id)
       assert leader |> Map.fetch!(:group_id) == group2.id
+
+      # test on update idempotence
+      usernames_and_groups2 = [
+        %{username: new_group_leader.user.username, group: "Existing Group"}
+      ]
+
+      assert :ok == Courses.upsert_groups_in_course(usernames_and_groups2, course2.id, "test")
+
+      group2 =
+        Group |> where(course_id: ^course2.id) |> where(name: "Existing Group") |> Repo.one()
+
+      assert group2 |> Map.fetch!(:leader_id) == new_group_leader.id
     end
   end