Failing multistream testcase

Would expect Identities, many-to-one, to project the view i into the 2nd Apply method.

src/EventSourcingTests/Projections/MultiStreamProjections/TestClasses.cs

@@ -135,4 +135,32 @@ public class UserGroupsAssignment
     public List<Guid> Groups { get; set; } = new();
 }
 
-#endregion
+#endregion
+
+
+// Notification example
+public class Notification
+{
+    public string Id { get; set; }
+    public string Title { get; set; }
+    public string Content { get; set; }
+
+    public Notification(string id, string title, string content)
+    {
+        Id = id;
+        Title = title;
+        Content = content;
+    }
+}
+
+public class UserNotification
+{
+    public string Id { get; set; }
+    public string UserId { get; set; }
+    public string NotificationId { get; set; }
+    public bool isOpened { get; set; }
+}
+
+// Notification events
+public record SendNotificationToUsers(Notification Notification, List<string> UserIds);
+public record UserOpenedNotification(string NotificationId, string UserId);

src/EventSourcingTests/Projections/MultiStreamProjections/simple_multi_stream_projection_wih_one_to_many_notification_many_first.cs

@@ -0,0 +1,81 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Marten.Events.Projections;
+using Marten.Testing.Harness;
+using Shouldly;
+using Xunit;
+
+namespace EventSourcingTests.Projections.MultiStreamProjections.Samples
+{
+    #region sample_view-projection-simple-with-one-to-many
+
+    public class UserNotificationProjection: MultiStreamProjection<UserNotification, string>
+    {
+        public UserNotificationProjection()
+        {
+            Identity<UserOpenedNotification>(x => x.UserId);
+            Identities<SendNotificationToUsers>(x => x.UserIds);
+        }
+
+        public void Apply(UserOpenedNotification @event, UserNotification view)
+        {
+            view.isOpened = true;
+        }
+
+        public void Apply(SendNotificationToUsers @event, UserNotification view)
+        {
+            Assert.NotNull(view.Id); //! Fails
+            view.Id = $"{view.Id}:{@event.Notification.Id}"; // I want the userNotification id to be "userId:notificationId", but since the matched userId is not available here, I can't do that.
+            view.NotificationId = @event.Notification.Id;
+        }
+    }
+
+    #endregion
+}
+
+namespace EventSourcingTests.Projections.MultiStreamProjections
+{
+
+
+    public class simple_multi_stream_projection_with_one_to_many_notification_many_first: OneOffConfigurationsContext
+    {
+        [Fact]
+        public async Task multi_stream_projections_with_only_group_assign_should_work()
+        {
+            // Create a notification and send it to John and Anna
+            var notification = new Notification(Guid.NewGuid().ToString(), "Notification title", "Example content");
+
+            var johnUserId = Guid.NewGuid().ToString(); // UserId of John
+            var annaUserId = Guid.NewGuid().ToString(); // UserId of Anna
+
+            var sendNotificationToUsers = new SendNotificationToUsers(notification, new List<string> { johnUserId, annaUserId });
+            theSession.Events.Append(sendNotificationToUsers.Notification.Id, sendNotificationToUsers);
+
+            await theSession.SaveChangesAsync(); //! FAILS because of inner assert
+
+            // Open the notification for John
+            var userOpenedNotification = new UserOpenedNotification(notification.Id, johnUserId);
+            theSession.Events.Append(userOpenedNotification.UserId, userOpenedNotification);
+
+            await theSession.SaveChangesAsync();
+
+            // Check if the notification is opened for John
+            var userNotification = await theSession.LoadAsync<UserNotification>($"{johnUserId}:{notification.Id}");
+            userNotification.isOpened.ShouldBeTrue();
+
+            // Check if the notification is not opened for Anna
+            userNotification = await theSession.LoadAsync<UserNotification>($"{annaUserId}:{notification.Id}");
+            userNotification.isOpened.ShouldBeFalse();
+        }
+
+        public simple_multi_stream_projection_with_one_to_many_notification_many_first()
+        {
+            StoreOptions(_ =>
+            {
+                _.Events.StreamIdentity = Marten.Events.StreamIdentity.AsString;
+                _.Projections.Add<EventSourcingTests.Projections.MultiStreamProjections.Samples.UserNotificationProjection>(ProjectionLifecycle.Inline);
+            });
+        }
+    }
+}