🐜 Fix syncing Courses with duplicate Number + Title (#79)

A bug was recently discovered where course data was missing for courses
that had identical titles and numbers, but different subjects: #76

As an example instance, in Spring 2025, MyPurdue lists `MA 41600` and
`STAT 41600`, both titled "Probability," however Purdue.io only shows
`MA 41600` with every CRN from both of the original courses grouped
under it.

The root cause is that CatalogSync caches courses indexed only by their
number and title, not by their subject. This causes any courses with an
identical number and title to be grouped together, even those from
different subjects.

This change modifies the index to key off of subject code in addition to
number and title, resolving the initial bug.

It also includes the logic needed to "move" classes that were
erroneously synced with the existing logic to the correct place.

Author: haydenmc

src/CatalogSync/FastSync.cs

@@ -87,8 +87,9 @@ public static async Task SynchronizeAsync(IScraper scraper, ApplicationDbContext
         private Dictionary<string, DatabaseCampus> dbCachedCampuses =
             new Dictionary<string, DatabaseCampus>();
 
-        private Dictionary<(string number, string title), DatabaseCourse> dbCachedCourses = 
-            new Dictionary<(string number, string title), DatabaseCourse>();
+        private Dictionary<(string subjectCode, string number, string title), DatabaseCourse>
+            dbCachedCourses = 
+                new Dictionary<(string subjectCode, string number, string title), DatabaseCourse>();
 
         private Dictionary<(Guid campusId, string buildingCode), DatabaseBuilding> dbCachedBuildings
             = new Dictionary<(Guid campusId, string buildingCode), DatabaseBuilding>();
@@ -374,6 +375,7 @@ private void InternalSynchronizeClass(DatabaseTerm term, DatabaseSubject subject
                 .ToList();
             if (dbSections.Count == 0)
             {
+                // The sections don't exist, so the class doesn't either.
                 var newClass = new DatabaseClass()
                 {
                     Id = Guid.NewGuid(),
@@ -386,7 +388,19 @@ private void InternalSynchronizeClass(DatabaseTerm term, DatabaseSubject subject
             }
             else
             {
+                // At least one of the sections exists in the database.
                 classId = dbSections.First().ClassId;
+
+                // Detect if the section's Class has moved to a different Course
+                // (handle the rare case where a CRN gets moved to a different Course)
+                var dbSectionGroupClass = dbContext.Classes.Single(c => c.Id == classId);
+                if (dbSectionGroupClass.CourseId != course.Id)
+                {
+                    dbSectionGroupClass.CourseId = course.Id;
+                    dbContext.Entry(dbSectionGroupClass)
+                        .Property(s => s.CourseId).CurrentValue = course.Id;
+                    dbContext.Entry(dbSectionGroupClass).State = EntityState.Modified;
+                }
             }
 
             // Hydrate each section
@@ -438,7 +452,8 @@ private DatabaseCourse FetchOrAddCourse(DatabaseSubject subject, string courseNu
             string courseTitle, double creditHours, string courseDescription)
         {
             DatabaseCourse course;
-            var courseKey = (number: courseNumber, title: courseTitle);
+            var courseKey = (subjectCode: subject.Abbreviation,
+                number: courseNumber, title: courseTitle);
             if (dbCachedCourses.ContainsKey(courseKey))
             {
                 course = dbCachedCourses[courseKey];