Enable re-using entities in enrichment from previous projection in a composition

src/DaemonTests/Composites/multi_stage_projections.cs

@@ -142,6 +142,7 @@ private async Task startAppointments()
 
         var boards = await _compositeSession.Query<Board>().ToListAsync();
 
+        var i = 0;
         foreach (var patient in patients)
         {
             var requested = new AppointmentRequested(patient.Id, faker.PickRandom(states),
@@ -150,14 +151,20 @@ private async Task startAppointments()
             var board = boards.FirstOrDefault(x =>
                 x.StateCodes.Contains(requested.StateCode) && x.SpecialtyCodes.Contains(requested.SpecialtyCode));
 
-            if (board != null)
+            var appointmentId = Guid.NewGuid();
+            List<object> events = [requested];
+            if (i % 2 == 0)
             {
-                _compositeSession.Events.StartStream<Appointment>(requested, new AppointmentRouted(board.Id));
+                events.Add(new AppointmentExternalIdentifierAssigned(appointmentId, Guid.NewGuid()));
             }
-            else
+            if (board != null)
             {
-                _compositeSession.Events.StartStream<Appointment>(requested);
+                events.Add(new AppointmentRouted(board.Id));
             }
+
+            _compositeSession.Events.StartStream<Appointment>(appointmentId, events);
+
+            i++;
         }
 
         await _compositeSession.SaveChangesAsync();
@@ -200,6 +207,7 @@ public async Task end_to_end(TenancyStyle tenancyStyle)
                 // 2nd stage projections
                 projection.Add<AppointmentDetailsProjection>(2);
                 projection.Add<BoardSummaryProjection>(2);
+                projection.Add<AppointmentByExternalIdentifierProjection>(2);
             });
 
             #endregion
@@ -235,6 +243,7 @@ public async Task end_to_end(TenancyStyle tenancyStyle)
 
         // Got details from the 2nd stage projection!
         (await _compositeSession.Query<AppointmentDetails>().CountAsync()).ShouldBeGreaterThan(0);
+        (await _compositeSession.Query<AppointmentByExternalIdentifier>().CountAsync()).ShouldBeGreaterThan(0);
 
         // See the downstream BoardSummary too!
         (await _compositeSession.Query<BoardSummary>().CountAsync()).ShouldBeGreaterThan(0);

src/DaemonTests/TeleHealth/AppointmentByExternalIdentifierProjection.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using JasperFx.Events;
+using JasperFx.Events.Grouping;
+using Marten;
+using Marten.Events.Projections;
+using Marten.Schema;
+
+namespace DaemonTests.TeleHealth;
+
+public class AppointmentByExternalIdentifierProjection : MultiStreamProjection<AppointmentByExternalIdentifier, Guid>
+{
+    public AppointmentByExternalIdentifierProjection()
+    {
+        Options.CacheLimitPerTenant = 1000;
+
+        Identity<AppointmentExternalIdentifierAssigned>(x => x.ExternalId);
+    }
+
+    public override async Task EnrichEventsAsync(SliceGroup<AppointmentByExternalIdentifier, Guid> group,
+        IQuerySession querySession, CancellationToken cancellation)
+    {
+        await group
+            .EnrichWith<Appointment>() // should be fetched from cache or from store
+            .ForEvent<AppointmentExternalIdentifierAssigned>()
+            .ForEntityId(x => x.AppointmentId)
+            .EnrichAsync((slice, @event, appointment) =>
+            {
+                slice.ReplaceEvent(@event, new EnrichedExternalIdentifierAssigned(@event.Data, appointment));
+            });
+    }
+
+    private sealed record EnrichedExternalIdentifierAssigned(AppointmentExternalIdentifierAssigned Assigned, Appointment Appointment);
+
+    public override AppointmentByExternalIdentifier Evolve(AppointmentByExternalIdentifier snapshot, Guid id, IEvent e) =>
+        e.Data switch
+        {
+            EnrichedExternalIdentifierAssigned enriched => new()
+            {
+                ExternalIdentifier = enriched.Assigned.ExternalId,
+                AppointmentId = enriched.Assigned.AppointmentId,
+                SpecialtyCode = enriched.Appointment.SpecialtyCode
+            },
+            _ => snapshot
+        };
+}
+
+public class AppointmentByExternalIdentifier
+{
+    [Identity]
+    public required Guid ExternalIdentifier { get; set; }
+    public required Guid AppointmentId { get; set; }
+
+    public required string SpecialtyCode { get; set; }
+}

src/DaemonTests/TeleHealth/AppointmentDetailsProjection.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 using JasperFx.Events;

src/DaemonTests/TeleHealth/Appointments.cs

@@ -6,6 +6,7 @@ namespace DaemonTests.TeleHealth;
 
 public record AppointmentRequested(Guid PatientId, string StateCode, string SpecialtyCode);
 public record AppointmentRouted(Guid BoardId);
+public record AppointmentExternalIdentifierAssigned(Guid AppointmentId, Guid ExternalId);
 public record ProviderAssigned( Guid ProviderId);
 public record AppointmentStarted;
 public record AppointmentCompleted;