Introduce Student Aggregate

StudentEnrollmentConsoleApp.sln

@@ -6,7 +6,6 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 	ProjectSection(SolutionItems) = preProject
 		docker-compose.yml = docker-compose.yml
 		.gitignore = .gitignore
-		.dockerignore = .dockerignore
 		global.json = global.json
 		README.md = README.md
 	EndProjectSection

StudentEnrollmentConsoleApp/Events/Event.cs

@@ -1,8 +1,6 @@
 namespace StudentEnrollmentConsoleApp.Events;
 
-public abstract class Event
+public abstract record Event
 {
-    public abstract string StreamId { get; }
-
-    public DateTime CreatedAtUtc { get; set; }
+    public string Id { get; init; } = default!;
 }

StudentEnrollmentConsoleApp/Events/EventTypeMapper.cs

@@ -0,0 +1,35 @@
+using System.Collections.Concurrent;
+
+namespace StudentEnrollmentConsoleApp.Events;
+
+public class EventTypeMapper
+{
+    public static readonly EventTypeMapper Instance = new();
+
+    private readonly ConcurrentDictionary<string, Type?> _typeMap = new();
+    private readonly ConcurrentDictionary<Type, string> _typeNameMap = new();
+
+    public string ToName<TEventType>() => ToName(typeof(TEventType));
+
+    public string ToName(Type eventType) => _typeNameMap.GetOrAdd(eventType, _ =>
+    {
+        var eventTypeName = eventType.FullName!;
+        _typeMap.TryAdd(eventTypeName, eventType);
+        return eventTypeName;
+    });
+
+    public Type? ToType(string eventTypeName) => _typeMap.GetOrAdd(eventTypeName, _ =>
+    {
+        var type = AppDomain.CurrentDomain
+            .GetAssemblies()
+            .SelectMany(a => a.GetTypes().Where(x => x.FullName == eventTypeName || x.Name == eventTypeName))
+            .FirstOrDefault();
+
+        if (type == null)
+            return null;
+
+        _typeNameMap.TryAdd(type, eventTypeName);
+
+        return type;
+    });
+}

StudentEnrollmentConsoleApp/Events/StudentCreated.cs

@@ -1,11 +1,9 @@
 namespace StudentEnrollmentConsoleApp.Events;
 
-public class StudentCreated : Event
+public record StudentCreated : Event
 {
-    public required string StudentId { get; init; }
     public required string FullName { get; init; }
     public required string Email { get; init; }
     public required DateTime DateOfBirth { get; init; }
-
-    public override string StreamId => StudentId;
+    public DateTime CreatedAtUtc { get; init; }
 }

StudentEnrollmentConsoleApp/Events/StudentEmailChanged.cs

@@ -0,0 +1,7 @@
+namespace StudentEnrollmentConsoleApp.Events;
+
+public record StudentEmailChanged : Event
+{
+    public required string Email { get; init; }
+    public DateTime ChangedAtUtc { get; init; }
+}

StudentEnrollmentConsoleApp/Events/StudentEnrolled.cs

@@ -1,9 +1,7 @@
 namespace StudentEnrollmentConsoleApp.Events;
 
-public class StudentEnrolled : Event
+public record StudentEnrolled : Event
 {
-    public required string StudentId { get; init; }
     public required string CourseName { get; init; }
-
-    public override string StreamId => StudentId;
+    public DateTime EnrolledAtUtc { get; init; }
 }

StudentEnrollmentConsoleApp/Events/StudentWithdrawn.cs

@@ -0,0 +1,7 @@
+namespace StudentEnrollmentConsoleApp.Events;
+
+public record StudentWithdrawn : Event
+{
+    public required string CourseName { get; init; }
+    public DateTime WithdrawnAtUtc { get; init; }
+}

StudentEnrollmentConsoleApp/Program.cs

@@ -1,58 +1,107 @@
 using System.Text;
 using System.Text.Json;
 using EventStore.Client;
-using StudentEnrollmentConsoleApp;
 using StudentEnrollmentConsoleApp.Events;
 
+// Register events to a singleton for ease-of-reference
+EventTypeMapper.Instance.ToName(typeof(StudentCreated));
+EventTypeMapper.Instance.ToName(typeof(StudentEnrolled));
+EventTypeMapper.Instance.ToName(typeof(StudentWithdrawn));
+EventTypeMapper.Instance.ToName(typeof(StudentEmailChanged));
+
 var id = Guid.Parse("a662d446-4920-415e-8c2a-0dd4a6c58908");
-var studentId = $"student-{id}";
-var now = DateTime.Now;
+var streamId = $"student-{id}";
 
-var studentCreated = new StudentCreated
-{
-    StudentId = studentId,
-    FullName = "Erik Shafer",
-    Email = "erik.shafer@eventstore.com",
-    DateOfBirth = new DateTime(1987, 1, 1)
-};
-const string eventType = nameof(studentCreated); //"StudentCreated";
-const string streamName = "some-stream";
+var created = new EventData(
+    Uuid.NewUuid(),
+    "StudentCreated",
+    JsonSerializer.SerializeToUtf8Bytes(new StudentCreated
+    {
+        Id = streamId,
+        FullName = "Erik Shafer",
+        Email = "erik.shafer@eventstore.com",
+        DateOfBirth = new DateTime(1987, 1, 1),
+        CreatedAtUtc = DateTime.UtcNow
+    })
+);
 
-var inMemoryDb = new StudentDatabase();
-inMemoryDb.Append(studentCreated);
+var enrolled = new EventData(
+    Uuid.NewUuid(),
+    "StudentEnrolled",
+    JsonSerializer.SerializeToUtf8Bytes(new StudentEnrolled
+    {
+        Id = streamId,
+        CourseName = "From Zero to Hero: REST APis in .NET",
+        EnrolledAtUtc = DateTime.UtcNow
+    })
+);
+
+var emailChanged = new EventData(
+    Uuid.NewUuid(),
+    "StudentEmailChanged",
+    JsonSerializer.SerializeToUtf8Bytes(new StudentEmailChanged
+    {
+        Id = streamId,
+        Email = "erik.shafer.changed.his.email@eventstore.com",
+        ChangedAtUtc = DateTime.UtcNow
+    })
+);
 
+// Our EventStoreDB (ESDB)
 const string connectionString = "esdb://admin:changeit@localhost:2113?tls=false&tlsVerifyCert=false";
 var settings = EventStoreClientSettings.Create(connectionString);
 var client = new EventStoreClient(settings);
 
-var eventData = new EventData(
-    Uuid.NewUuid(),
-    eventType,
-    JsonSerializer.SerializeToUtf8Bytes(studentCreated)
-);
-
+// Append to ESDB
 await client.AppendToStreamAsync(
-    streamName,
+    streamId,
     StreamState.Any,
-    new[] { eventData },
+    new[] { created, enrolled, emailChanged },
     cancellationToken: default
 );
 
-var result = client.ReadStreamAsync(
+// Read from ESDB
+var readStreamResult = client.ReadStreamAsync(
     Direction.Forwards,
-    streamName,
+    streamId,
     StreamPosition.Start,
     cancellationToken: default
 );
+var eventStream = await readStreamResult.ToListAsync();
 
-var events = await result.ToListAsync();
+// Write out the events from the stream
+Console.WriteLine("Events from selected stream: ");
+foreach (var resolved in eventStream)
+{
+    Console.WriteLine($"\tEventId: {resolved.Event.EventId}");
+    Console.WriteLine($"\tEventStreamId: {resolved.Event.EventStreamId}");
+    Console.WriteLine($"\tEventType: {resolved.Event.EventType}");
+    Console.WriteLine($"\tData: {Encoding.UTF8.GetString(resolved.Event.Data.ToArray())}");
+    Console.WriteLine("");
+}
 
-foreach (var @event in events)
+// Write out all the courses the student enrolled in
+var enrolledCourses = eventStream
+    .Where(re => re.Event.EventType == "StudentEnrolled")
+    .Select(re => JsonSerializer.Deserialize<StudentEnrolled>(re.Event.Data.ToArray()))
+    .Select(se => se!.CourseName)
+    .ToList();
+Console.WriteLine("Courses enrolled in: ");
+enrolledCourses.ForEach(ec => Console.WriteLine($"\t- {ec}"));
+Console.WriteLine("");
+
+// Write out using the mapper
+Console.WriteLine("Deserialized events:");
+foreach (var resolved in eventStream)
 {
-    Console.WriteLine($"EventType: {@event.Event.EventId}");
-    Console.WriteLine($"EventStreamId: {@event.Event.EventStreamId}");
-    Console.WriteLine($"EventType: {@event.Event.EventType}");
-    var data = Encoding.UTF8.GetString(@event.Event.Data.ToArray());
-    Console.WriteLine($"Data: {data}");
-    Console.WriteLine("-----");
+    var eventType = EventTypeMapper.Instance.ToType(resolved.Event.EventType);
+
+    if (eventType == null)
+        break;
+
+    var deserializedEvent = JsonSerializer.Deserialize(Encoding.UTF8.GetString(resolved.Event.Data.Span), eventType);
+
+    Console.WriteLine($"\t{deserializedEvent}");
 }
+
+Console.WriteLine("");

StudentEnrollmentConsoleApp/Student.cs

@@ -0,0 +1,57 @@
+using StudentEnrollmentConsoleApp.Events;
+
+namespace StudentEnrollmentConsoleApp;
+
+public class Student
+{
+    public string Id { get; set; } = default!;
+    public string FullName { get; set; } = default!;
+    public string Email { get; set; } = default!;
+    public DateTime DateOfBirth { get; set; }
+    public DateTime CreatedAtUtc { get; set; }
+    public List<string> EnrolledCourses { get; set; } = [];
+
+    public void Apply(Event @event)
+    {
+        switch (@event)
+        {
+            case StudentCreated created:
+                Apply(created);
+                break;
+            case StudentEmailChanged emailChanged:
+                Apply(emailChanged);
+                break;
+            case StudentEnrolled enrolled:
+                Apply(enrolled);
+                break;
+            case StudentWithdrawn withdrawn:
+                Apply(withdrawn);
+                break;
+        }
+    }
+
+    private void Apply(StudentCreated @event)
+    {
+        Id = @event.Id;
+        FullName = @event.FullName;
+        Email = @event.Email;
+        CreatedAtUtc = @event.CreatedAtUtc;
+    }
+
+    private void Apply(StudentEmailChanged @event)
+    {
+        Email = @event.Email;
+    }
+
+    private void Apply(StudentEnrolled @event)
+    {
+        if (EnrolledCourses.Contains(@event.CourseName) is false)
+            EnrolledCourses.Add(@event.CourseName);
+    }
+
+    private void Apply(StudentWithdrawn @event)
+    {
+        if (EnrolledCourses.Contains(@event.CourseName))
+            EnrolledCourses.Add(@event.CourseName);
+    }
+}