Patch for #4225

[zehata]

@leslieyip02 @jloh02 Patch for #4225, for the issue reported on the morning of 16 Dec by LH (https://t.me/NUSMods/13136)

Important Note: While this patch preserves the current behavior of the app, I think it is more prudent to add more error checking to the migration process, and to stop the migration if an error is encountered, because migrating the config is an inherently destructive process (it overwrites the old config). I am currently working on that. This patch should be considered superseded by #4272

1. When the app fetches the timetable modules, it populates the lesson indices which allows the migration of the module config to the new schema
   

   nusmods/website/src/reducers/moduleBank.ts

   Lines 47 to 60 in d31261e

   	case SUCCESS_KEY(FETCH_MODULE):
   	return {
   	...state,
   	modules: {
   	...state.modules,
   	[action.payload.moduleCode]: {
   	...action.payload,
   	timestamp: Date.now(),
   	semesterData: map(action.payload.semesterData, (semesterData) => ({
   	...semesterData,
   	timetable: map(semesterData.timetable, (lesson, lessonIndex) => ({
   	...lesson,
   	lessonIndex,
   	})),

2. The issue was caused by
   

   nusmods/website/src/views/AppShell.tsx

   Lines 64 to 66 in d31261e

   	function fetchTimetableModulesImpl(timetable: SemTimetableConfig, semester: Semester) {
   	dispatch(fetchTimetableModulesAction([timetable]))
   	.then(() => dispatch(validateTimetable(semester)))

   
   calling
   

   nusmods/website/src/actions/timetables.ts

   Lines 231 to 239 in d31261e

   	export function validateTimetable(semester: Semester) {
   	return (dispatch: Dispatch, getState: GetState) => {
   	const { timetables, moduleBank } = getState();
   	const { lessons, ta, alreadyMigrated } = migrateTimetableConfigs(
   	timetables.lessons as TimetableConfig | TimetableConfigV1,
   	timetables.ta as TaModulesMap | TaModulesMapV1,
   	moduleBank.modules,
   	);

When validateTimetable(1) is called when the app successfully fetches the module data for semester 1, it calls migrateTimetableConfigs which migrates the configs for all semesters, including semester 2. Since the request for semester 2 has not returned, the lesson indices has not been populated for semester 2 (see point 1). Trying to migrate the config for semester 2 will thus fail.

This patch fixes this issue by removing the migrateTimetableConfigs function and instead migrates each semester's timetable config as data is fetched for each of the semester.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.58%. Comparing base (988c6fd) to head (9b97038).
⚠️ Report is 152 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4271      +/-   ##
==========================================
+ Coverage   54.52%   56.58%   +2.05%     
==========================================
  Files         274      297      +23     
  Lines        6076     6917     +841     
  Branches     1455     1667     +212     
==========================================
+ Hits         3313     3914     +601     
- Misses       2763     3003     +240     

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