refactor(classes): preserve student membership history

apps/server/src/classes/classes.membership.spec.ts

@@ -0,0 +1,95 @@
+import { BadRequestException, NotFoundException } from '@nestjs/common';
+import { ClassesService } from './classes.service';
+import { ClassStudent } from '../entities';
+
+function createService(
+  classStudentRepo: Record<string, jest.Mock>,
+  classRepo: Record<string, jest.Mock> = { findOne: jest.fn().mockResolvedValue({ id: 3 }) },
+  studentRepo: Record<string, jest.Mock> = {
+    find: jest.fn().mockResolvedValue([{ id: 8 }, { id: 9 }, { id: 10 }]),
+  },
+) {
+  return new ClassesService(
+    classRepo as never,
+    classStudentRepo as never,
+    {} as never,
+    {} as never,
+    {} as never,
+    {} as never,
+    studentRepo as never,
+    {} as never,
+  );
+}
+
+describe('ClassesService — student membership lifecycle', () => {
+  it('marks an active membership as left instead of deleting it', async () => {
+    const membership = {
+      classId: 3,
+      studentId: 8,
+      status: 'active',
+      leaveDate: null,
+    } as ClassStudent;
+    const repo = {
+      findOne: jest.fn().mockResolvedValue(membership),
+      save: jest.fn().mockImplementation(async (value: ClassStudent) => value),
+    };
+    const service = createService(repo);
+
+    await service.removeStudent(3, 8);
+
+    expect(membership.status).toBe('left');
+    expect(membership.leaveDate).toMatch(/^\d{4}-\d{2}-\d{2}$/);
+    expect(repo.save).toHaveBeenCalledWith(membership);
+  });
+
+  it('rejects removing an already-left membership', async () => {
+    const repo = {
+      findOne: jest.fn().mockResolvedValue({ status: 'left' }),
+      save: jest.fn(),
+    };
+    const service = createService(repo);
+
+    await expect(service.removeStudent(3, 8)).rejects.toBeInstanceOf(BadRequestException);
+    expect(repo.save).not.toHaveBeenCalled();
+  });
+
+  it('rejects removing a student without a membership', async () => {
+    const repo = {
+      findOne: jest.fn().mockResolvedValue(null),
+      save: jest.fn(),
+    };
+    const service = createService(repo);
+
+    await expect(service.removeStudent(3, 8)).rejects.toBeInstanceOf(NotFoundException);
+  });
+
+  it('reactivates left memberships, creates new ones, and skips active ones', async () => {
+    const left = {
+      id: 1,
+      classId: 3,
+      studentId: 8,
+      status: 'left',
+      joinDate: '2026-01-01',
+      leaveDate: '2026-02-01',
+    } as ClassStudent;
+    const active = { id: 2, classId: 3, studentId: 9, status: 'active' } as ClassStudent;
+    const repo = {
+      find: jest.fn().mockResolvedValue([left, active]),
+      create: jest.fn().mockImplementation((value: Partial<ClassStudent>) => value),
+      save: jest.fn().mockImplementation(async (value: ClassStudent[]) => value),
+    };
+    const service = createService(repo);
+
+    const result = await service.addStudents(3, [8, 9, 10, 10]);
+
+    expect(result).toEqual({ added: 2, skipped: 1 });
+    expect(left).toMatchObject({ status: 'active', leaveDate: null });
+    expect(left.joinDate).toMatch(/^\d{4}-\d{2}-\d{2}$/);
+    expect(repo.create).toHaveBeenCalledWith(
+      expect.objectContaining({ classId: 3, studentId: 10, status: 'active' }),
+    );
+    expect(repo.save).toHaveBeenCalledWith(
+      expect.arrayContaining([left, expect.objectContaining({ studentId: 10 })]),
+    );
+  });
+});