refactor(classes): preserve student membership history

apps/server/src/database/database-migrations.class-student.spec.ts

@@ -0,0 +1,56 @@
+import { Test, TestingModule } from '@nestjs/testing';
+import { getDataSourceToken } from '@nestjs/typeorm';
+import { DatabaseMigrationsService } from './database-migrations.service';
+
+function createRunner(tableExists: boolean, columns: string[] = []) {
+  return {
+    connect: jest.fn(),
+    release: jest.fn(),
+    getTables: jest.fn().mockResolvedValue(tableExists ? [{ name: 'class_student' }] : []),
+    getTable: jest.fn().mockResolvedValue({
+      name: 'class_student',
+      columns: columns.map((name) => ({ name })),
+    }),
+    dropColumn: jest.fn().mockResolvedValue(undefined),
+  };
+}
+
+async function createService(runner: ReturnType<typeof createRunner>) {
+  const module: TestingModule = await Test.createTestingModule({
+    providers: [
+      DatabaseMigrationsService,
+      {
+        provide: getDataSourceToken(),
+        useValue: {
+          options: { type: 'better-sqlite3' },
+          createQueryRunner: jest.fn().mockReturnValue(runner),
+        },
+      },
+    ],
+  }).compile();
+  return module.get(DatabaseMigrationsService) as DatabaseMigrationsService & {
+    removeUnusedClassStudentColumns(): Promise<void>;
+  };
+}
+
+describe('DatabaseMigrationsService — class student cleanup', () => {
+  it('drops the unused enrollment_id column', async () => {
+    const runner = createRunner(true, ['id', 'enrollment_id']);
+    const service = await createService(runner);
+
+    await service.removeUnusedClassStudentColumns();
+
+    expect(runner.dropColumn).toHaveBeenCalledWith('class_student', 'enrollment_id');
+    expect(runner.release).toHaveBeenCalled();
+  });
+
+  it('does nothing when the table is absent', async () => {
+    const runner = createRunner(false);
+    const service = await createService(runner);
+
+    await service.removeUnusedClassStudentColumns();
+
+    expect(runner.dropColumn).not.toHaveBeenCalled();
+    expect(runner.release).toHaveBeenCalled();
+  });
+});