Fix CourseMembership.clean() crash on unsaved course & enforce teacher requirement

[smattymatty]

Fix CourseMembership.clean() crash on unsaved course & enforce teacher requirement

Description

Fixes two related issues with course creation and membership validation in the Django admin.

Closes #260

Mandatory Checklist

• I have read and followed the CONTRIBUTING.md
• I have linked the related issue above
• My branch is up to date with main
• I have tested my changes locally
• I have added or updated tests where applicable
• I have not introduced any new linting errors
• My commit messages are clear and descriptive
• I have updated documentation if my changes affect public APIs, setup steps, or user-facing behavior

What Changed

1. Guard CourseMembership.clean() against unsaved instances (courses/models.py)

The duplicate-membership check ran CourseMembership.objects.filter(user=self.user, course=self.course) unconditionally. When creating a course with an inline membership in the admin, self.course has no PK yet, causing Django to raise ValueError: Model instances passed to related filters must be saved.

Fixed by wrapping the query in if self.user_id and self.course_id: — skips the DB query when either FK is unsaved. The UniqueConstraint on ["user", "course"] still prevents duplicates at the DB level.

2. Enforce teacher requirement in admin (courses/admin.py)

Added CourseAdmin.save_related() override that checks after all inlines are saved that the course has at least one CourseMembership with role="teacher" and status="enrolled". Raises ValidationError if not, preventing:

• Creating courses with no teacher
• Removing the last teacher via admin

The API flow (auto-creating teacher membership on course creation) is unchanged.

3. Tests

• courses/tests/test_CourseMembership.py — 3 new tests: unsaved course doesn't crash, unsaved user doesn't crash, duplicates still caught when both are saved
• courses/tests/test_admin.py — new file, 5 tests: no teacher blocked, with teacher succeeds, only students blocked, invited teacher doesn't count, removing last teacher blocked

How to Test

cd backend/EduLite
python manage.py test courses

All 1020 backend tests pass (157 in courses app, 8 new).

To manually verify:

1. Open Django admin → Courses → Add Course
2. Try saving without adding a membership inline → should show validation error
3. Add a teacher membership inline → should save successfully
4. Edit the course, delete the teacher membership → should show validation error

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Project	Deployment	Actions	Updated (UTC)
edulite	Ready	Preview, Comment	Feb 13, 2026 3:55pm

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