feat(event-subscriber): enrich the projection with the zaak reference via the ACL (refs #78)

On each notification the subscriber reads the zaak's reference (identificatie)
through the ACL — the only code allowed to talk to ZGW (§8.1) — and persists it on
the register_projection row and in the processed_notifications replay log. Storing
it in the log keeps rebuild log-only (ADR-0008): no ACL/ZGW access on rebuild.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/event-subscriber/EventSubscriber.Api/AclHttpClient.cs

@@ -0,0 +1,30 @@
+using System.Net.Http.Json;
+using System.Text.Json.Serialization;
+using EventSubscriber.Application;
+
+namespace EventSubscriber.Api;
+
+/// <summary>
+/// HTTP client to the ACL service. The subscriber enriches the projection with the zaak's reference
+/// (identificatie) by asking the ACL — the only code that may read ZGW (§8.1) — rather than reading
+/// OpenZaak itself (adr-proposal #78).
+/// </summary>
+public sealed class AclHttpClient(HttpClient http) : IAclClient
+{
+    public async Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(zaakUrl);
+
+        using var response = await http.PostAsJsonAsync(
+            new Uri(http.BaseAddress!, "zaken/reference"), new ReferenceRequest(zaakUrl.ToString()), ct);
+        response.EnsureSuccessStatusCode();
+
+        var body = await response.Content.ReadFromJsonAsync<ReferenceResponse>(ct)
+            ?? throw new InvalidOperationException("The ACL returned an empty reference response.");
+        return body.Reference;
+    }
+
+    private sealed record ReferenceRequest([property: JsonPropertyName("zaakUrl")] string ZaakUrl);
+
+    private sealed record ReferenceResponse([property: JsonPropertyName("reference")] string Reference);
+}

services/event-subscriber/EventSubscriber.Api/Program.cs

@@ -11,9 +11,15 @@ var connectionString = builder.Configuration.GetConnectionString("Projection")
 // this value (ADR-0007), so the webhook enforces it.
 var webhookToken = builder.Configuration["EventSubscriber:Webhook:AuthToken"]
     ?? throw new InvalidOperationException("Missing configuration 'EventSubscriber:Webhook:AuthToken'");
+// The ACL is the only code that may read ZGW (§8.1); the subscriber enriches the projection with the
+// zaak reference through it (adr-proposal #78).
+var aclBaseUrl = builder.Configuration["Acl:BaseUrl"]
+    ?? throw new InvalidOperationException("Missing configuration 'Acl:BaseUrl'");
 
 builder.Services.AddProjectionReadModel(connectionString);
 builder.Services.AddProjectionWriteSide();
+builder.Services.AddHttpClient<EventSubscriber.Application.IAclClient, EventSubscriber.Api.AclHttpClient>(
+    c => c.BaseAddress = new Uri(aclBaseUrl));
 
 var app = builder.Build();
 

services/event-subscriber/EventSubscriber.Application/Notification.cs

@@ -24,10 +24,12 @@ public sealed record Notification(
     public bool IsZaakStatusSet =>
         Kanaal == "zaken" && Resource == "status" && Actie == "create";
 
-    /// <summary>The zaak UUID used as the projection key. For a status notification the resource URL is
-    /// the status, so the zaak comes from <c>hoofdObject</c> (which, for a zaak-create, equals the
-    /// resource URL) — see the NRC payload note.</summary>
-    public string ZaakId => (HoofdObject ?? ResourceUrl).Segments[^1].Trim('/');
+    /// <summary>The zaak URL this notification concerns — <c>hoofdObject</c> (the zaak) for a status
+    /// notification, else the resource URL (which, for a zaak-create, is the zaak).</summary>
+    public Uri ZaakUrl => HoofdObject ?? ResourceUrl;
+
+    /// <summary>The zaak UUID used as the projection key — the trailing segment of <see cref="ZaakUrl"/>.</summary>
+    public string ZaakId => ZaakUrl.Segments[^1].Trim('/');
 
     /// <summary>
     /// A deterministic dedup key. Open Notificaties carries no notification id and may

services/event-subscriber/EventSubscriber.Application/NotificationProjector.cs

@@ -5,17 +5,19 @@ namespace EventSubscriber.Application;
 /// out-of-order deliveries (CLAUDE.md §8.6): the notification log dedups, and the projection
 /// upsert is idempotent on the zaak id. Rebuilds the projection by replaying the log.
 /// </summary>
-public sealed class NotificationProjector(INotificationLog log, IProjectionStore store)
+public sealed class NotificationProjector(INotificationLog log, IProjectionStore store, IAclClient acl)
 {
     /// <summary>Handle one inbound notification. Reacts to a zaak being created (INGEDIEND) and a
-    /// status being set (INGESCHREVEN); ignores everything else.</summary>
+    /// status being set (INGESCHREVEN); ignores everything else. Enriches the row with the zaak's
+    /// reference via the ACL (§8.1) and records it so a rebuild needs no ZGW access (#78).</summary>
     public async Task HandleAsync(Notification notification, CancellationToken ct = default)
     {
         if (!notification.IsZaakCreated && !notification.IsZaakStatusSet)
             return;
 
+        var reference = await acl.GetZaakReferenceAsync(notification.ZaakUrl, ct);
         var recorded = new RecordedNotification(
-            notification.IdempotencyKey, notification.Actie, notification.ZaakId, notification.Resource);
+            notification.IdempotencyKey, notification.Actie, notification.ZaakId, notification.Resource, reference);
 
         // Atomic record-or-skip: a duplicate (or concurrent) delivery is recognised and dropped
         // before it touches the projection, so the projection stays a faithful derived artefact.
@@ -36,7 +38,8 @@ public sealed class NotificationProjector(INotificationLog log, IProjectionStore
     /// <summary>The projection row for an accepted notification: a status-set maps to INGESCHREVEN,
     /// a zaak-create to INGEDIEND. bsn/naam are deferred (ADR-0008).</summary>
     private static RegisterEntry ToEntry(RecordedNotification recorded)
-        => new(recorded.ZaakId, recorded.Resource == "status"
-            ? RegistrationStatus.Ingeschreven
-            : RegistrationStatus.Ingediend);
+        => new(
+            recorded.ZaakId,
+            recorded.Resource == "status" ? RegistrationStatus.Ingeschreven : RegistrationStatus.Ingediend,
+            Reference: recorded.Reference);
 }

services/event-subscriber/EventSubscriber.Application/Ports.cs

@@ -20,9 +20,20 @@ public interface INotificationLog
 }
 
 /// <summary>A notification that has been accepted, retaining what a rebuild needs to recompute its
-/// projection row — including the ZGW <c>resource</c>, which distinguishes a zaak-create (INGEDIEND)
-/// from a status-set (INGESCHREVEN) so a rebuild reproduces the right status.</summary>
-public sealed record RecordedNotification(string Key, string Actie, string ZaakId, string Resource);
+/// projection row — the ZGW <c>resource</c> (zaak-create → INGEDIEND vs status-set → INGESCHREVEN) and
+/// the zaak <c>reference</c> (identificatie), so a rebuild reproduces the row without re-reading ZGW (#78).</summary>
+public sealed record RecordedNotification(string Key, string Actie, string ZaakId, string Resource, string? Reference);
+
+/// <summary>
+/// Port to the Anti-Corruption Layer. The subscriber enriches the projection with the zaak's
+/// public-safe reference (its identificatie) by asking the ACL — the only code that may read ZGW
+/// (§8.1) — rather than reading OpenZaak itself (adr-proposal #78).
+/// </summary>
+public interface IAclClient
+{
+    /// <summary>The zaak's reference (identificatie) for the read projection.</summary>
+    Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default);
+}
 
 /// <summary>The read projection store. Owned by the projection bounded context (ADR-0008); the
 /// subscriber writes to it and the projection-api reads it.</summary>

services/event-subscriber/EventSubscriber.Application/RegisterEntry.cs

@@ -9,6 +9,7 @@ namespace EventSubscriber.Application;
 public sealed record RegisterEntry(
     string Id,
     string Status,
+    string? Reference = null,
     string? Bsn = null,
     string? NaamPlaceholder = null);
 

services/event-subscriber/EventSubscriber.Tests/AclHttpClientTests.cs

@@ -0,0 +1,86 @@
+using System.Net;
+using System.Text;
+using EventSubscriber.Api;
+
+namespace EventSubscriber.Tests;
+
+/// <summary>
+/// Unit tests for the subscriber's ACL client, which reads a zaak's reference (identificatie) through
+/// the ACL — the only code allowed to talk to ZGW (§8.1, #78). Uses a scripted message handler so no
+/// real ACL is required.
+/// </summary>
+public class AclHttpClientTests
+{
+    private static AclHttpClient Client(StubHandler handler) =>
+        new(new HttpClient(handler) { BaseAddress = new Uri("http://acl/") });
+
+    [Fact]
+    public async Task Reads_a_zaak_reference_by_posting_the_zaak_url_and_returns_it()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.OK, """{"reference":"REG-42"}"""));
+
+        var reference = await client.GetZaakReferenceAsync(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+
+        Assert.Equal("REG-42", reference);
+        Assert.Equal(HttpMethod.Post, capture.Seen!.Method);
+        Assert.Equal("http://acl/zaken/reference", capture.Seen.RequestUri!.ToString());
+        Assert.Contains("\"zaakUrl\":\"http://openzaak/zaken/api/v1/zaken/abc\"", capture.Body);
+    }
+
+    [Fact]
+    public async Task Throws_when_the_acl_rejects_the_request()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.BadGateway));
+
+        await Assert.ThrowsAsync<HttpRequestException>(
+            () => client.GetZaakReferenceAsync(new Uri("http://openzaak/zaken/api/v1/zaken/abc")));
+    }
+
+    [Fact]
+    public async Task Throws_when_the_acl_returns_an_empty_body()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.OK, "null"));
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => client.GetZaakReferenceAsync(new Uri("http://openzaak/zaken/api/v1/zaken/abc")));
+        Assert.Contains("empty", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public async Task Rejects_a_null_zaak_url_without_sending_a_request()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.OK, """{"reference":"REG-1"}"""));
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => client.GetZaakReferenceAsync(null!));
+        Assert.Null(capture.Seen);
+    }
+}
+
+/// <summary>A test double for <see cref="HttpMessageHandler"/> that records the last request and
+/// returns a scripted response — mirrors the ACL/domain gateway tests.</summary>
+internal sealed class StubHandler(Func<HttpRequestMessage, Task<HttpResponseMessage>> onSend) : HttpMessageHandler
+{
+    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken ct)
+        => onSend(request);
+}
+
+/// <summary>Captures the request a client sent, including the (buffered) body.</summary>
+internal sealed class RequestCapture
+{
+    public HttpRequestMessage? Seen { get; private set; }
+    public string? Body { get; private set; }
+
+    public StubHandler Responds(HttpStatusCode status, string? json = null) => new(async req =>
+    {
+        Seen = req;
+        Body = req.Content is null ? null : await req.Content.ReadAsStringAsync();
+        var response = new HttpResponseMessage(status);
+        if (json is not null)
+            response.Content = new StringContent(json, Encoding.UTF8, "application/json");
+        return response;
+    });
+}

services/event-subscriber/EventSubscriber.Tests/EventSubscriber.Tests.csproj

@@ -19,6 +19,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <ProjectReference Include="..\EventSubscriber.Api\EventSubscriber.Api.csproj" />
     <ProjectReference Include="..\EventSubscriber.Application\EventSubscriber.Application.csproj" />
   </ItemGroup>
 

services/event-subscriber/EventSubscriber.Tests/InMemoryStores.cs

@@ -5,6 +5,19 @@ namespace EventSubscriber.Tests;
 /// <summary>In-memory stand-ins for the projection store and notification log, so the
 /// projector's behaviour is exercised without Postgres (hand-written stubs, the repo's
 /// convention — no mocking library).</summary>
+/// <summary>A fake ACL client that returns a fixed reference derived from the zaak, and records
+/// how many times it was called (to prove a rebuild does not re-read via the ACL).</summary>
+internal sealed class FakeAclClient : IAclClient
+{
+    public int CallCount { get; private set; }
+
+    public Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        CallCount++;
+        return Task.FromResult("REG-" + zaakUrl.Segments[^1].Trim('/'));
+    }
+}
+
 internal sealed class InMemoryNotificationLog : INotificationLog
 {
     private readonly Dictionary<string, RecordedNotification> _byKey = [];

services/event-subscriber/EventSubscriber.Tests/NotificationProjectorTests.cs

@@ -12,8 +12,9 @@ public sealed class NotificationProjectorTests
 
     private readonly InMemoryNotificationLog _log = new();
     private readonly InMemoryProjectionStore _store = new();
+    private readonly FakeAclClient _acl = new();
 
-    private NotificationProjector Projector() => new(_log, _store);
+    private NotificationProjector Projector() => new(_log, _store, _acl);
 
     private static Notification ZaakCreated(string url = ZaakUrl)
         => new("zaken", "zaak", "create", new Uri(url));
@@ -30,6 +31,23 @@ public sealed class NotificationProjectorTests
         var entry = Assert.Single(await _store.AllAsync());
         Assert.Equal("11111111-1111-1111-1111-111111111111", entry.Id);
         Assert.Equal(RegistrationStatus.Ingediend, entry.Status);
+        // Enriched with the zaak's reference (identificatie), fetched via the ACL (#78).
+        Assert.Equal("REG-11111111-1111-1111-1111-111111111111", entry.Reference);
+    }
+
+    [Fact]
+    public async Task rebuild_reproduces_the_reference_without_re_reading_via_the_acl()
+    {
+        var projector = Projector();
+        await projector.HandleAsync(ZaakCreated());
+        var callsAfterProjection = _acl.CallCount;
+
+        await projector.RebuildAsync();
+
+        var entry = Assert.Single(await _store.AllAsync());
+        Assert.Equal("REG-11111111-1111-1111-1111-111111111111", entry.Reference);
+        // Rebuild replays the log (which stored the reference) — no extra ACL calls (#78, ADR-0008).
+        Assert.Equal(callsAfterProjection, _acl.CallCount);
     }
 
     [Fact]

services/projection-api/Projection.ReadModel/EfNotificationLog.cs

@@ -16,6 +16,7 @@ public sealed class EfNotificationLog(ProjectionDbContext db) : INotificationLog
             Actie = notification.Actie,
             ZaakId = notification.ZaakId,
             Resource = notification.Resource,
+            Reference = notification.Reference,
             ReceivedAt = DateTimeOffset.UtcNow,
         });
 
@@ -35,6 +36,6 @@ public sealed class EfNotificationLog(ProjectionDbContext db) : INotificationLog
     public async Task<IReadOnlyList<RecordedNotification>> AllAsync(CancellationToken ct = default)
         => await db.ProcessedNotifications
             .OrderBy(r => r.ReceivedAt)
-            .Select(r => new RecordedNotification(r.Key, r.Actie, r.ZaakId, r.Resource))
+            .Select(r => new RecordedNotification(r.Key, r.Actie, r.ZaakId, r.Resource, r.Reference))
             .ToListAsync(ct);
 }

services/projection-api/Projection.ReadModel/EfProjectionStore.cs

@@ -15,6 +15,7 @@ public sealed class EfProjectionStore(ProjectionDbContext db) : IProjectionStore
             {
                 Id = entry.Id,
                 Status = entry.Status,
+                Reference = entry.Reference,
                 Bsn = entry.Bsn,
                 NaamPlaceholder = entry.NaamPlaceholder,
             });
@@ -22,6 +23,7 @@ public sealed class EfProjectionStore(ProjectionDbContext db) : IProjectionStore
         else
         {
             row.Status = entry.Status;
+            row.Reference = entry.Reference;
             row.Bsn = entry.Bsn;
             row.NaamPlaceholder = entry.NaamPlaceholder;
         }
@@ -35,6 +37,6 @@ public sealed class EfProjectionStore(ProjectionDbContext db) : IProjectionStore
     public async Task<IReadOnlyList<RegisterEntry>> AllAsync(CancellationToken ct = default)
         => await db.RegisterEntries
             .OrderBy(r => r.Id)
-            .Select(r => new RegisterEntry(r.Id, r.Status, r.Bsn, r.NaamPlaceholder))
+            .Select(r => new RegisterEntry(r.Id, r.Status, Reference: r.Reference, Bsn: r.Bsn, NaamPlaceholder: r.NaamPlaceholder))
             .ToListAsync(ct);
 }

services/projection-api/Projection.ReadModel/Migrations/20260714101642_AddReferenceColumns.Designer.cs

@@ -0,0 +1,92 @@
+// <auto-generated />
+using System;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+using Projection.ReadModel;
+
+#nullable disable
+
+namespace Projection.ReadModel.Migrations
+{
+    [DbContext(typeof(ProjectionDbContext))]
+    [Migration("20260714101642_AddReferenceColumns")]
+    partial class AddReferenceColumns
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder
+                .HasAnnotation("ProductVersion", "10.0.0")
+                .HasAnnotation("Relational:MaxIdentifierLength", 63);
+
+            NpgsqlModelBuilderExtensions.UseIdentityByDefaultColumns(modelBuilder);
+
+            modelBuilder.Entity("Projection.ReadModel.ProcessedNotificationRow", b =>
+                {
+                    b.Property<string>("Key")
+                        .HasColumnType("text")
+                        .HasColumnName("key");
+
+                    b.Property<string>("Actie")
+                        .IsRequired()
+                        .HasColumnType("text")
+                        .HasColumnName("actie");
+
+                    b.Property<DateTimeOffset>("ReceivedAt")
+                        .HasColumnType("timestamp with time zone")
+                        .HasColumnName("received_at");
+
+                    b.Property<string>("Reference")
+                        .HasColumnType("text")
+                        .HasColumnName("reference");
+
+                    b.Property<string>("Resource")
+                        .IsRequired()
+                        .HasColumnType("text")
+                        .HasColumnName("resource");
+
+                    b.Property<string>("ZaakId")
+                        .IsRequired()
+                        .HasColumnType("text")
+                        .HasColumnName("zaak_id");
+
+                    b.HasKey("Key");
+
+                    b.ToTable("processed_notifications", (string)null);
+                });
+
+            modelBuilder.Entity("Projection.ReadModel.RegisterEntryRow", b =>
+                {
+                    b.Property<string>("Id")
+                        .HasColumnType("text")
+                        .HasColumnName("id");
+
+                    b.Property<string>("Bsn")
+                        .HasColumnType("text")
+                        .HasColumnName("bsn");
+
+                    b.Property<string>("NaamPlaceholder")
+                        .HasColumnType("text")
+                        .HasColumnName("naam_placeholder");
+
+                    b.Property<string>("Reference")
+                        .HasColumnType("text")
+                        .HasColumnName("reference");
+
+                    b.Property<string>("Status")
+                        .IsRequired()
+                        .HasColumnType("text")
+                        .HasColumnName("status");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("register_projection", (string)null);
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

services/projection-api/Projection.ReadModel/Migrations/20260714101642_AddReferenceColumns.cs

@@ -0,0 +1,38 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Projection.ReadModel.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddReferenceColumns : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "reference",
+                table: "register_projection",
+                type: "text",
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "reference",
+                table: "processed_notifications",
+                type: "text",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "reference",
+                table: "register_projection");
+
+            migrationBuilder.DropColumn(
+                name: "reference",
+                table: "processed_notifications");
+        }
+    }
+}

services/projection-api/Projection.ReadModel/Migrations/ProjectionDbContextModelSnapshot.cs

@@ -37,6 +37,10 @@ namespace Projection.ReadModel.Migrations
                         .HasColumnType("timestamp with time zone")
                         .HasColumnName("received_at");
 
+                    b.Property<string>("Reference")
+                        .HasColumnType("text")
+                        .HasColumnName("reference");
+
                     b.Property<string>("Resource")
                         .IsRequired()
                         .HasColumnType("text")
@@ -66,6 +70,10 @@ namespace Projection.ReadModel.Migrations
                         .HasColumnType("text")
                         .HasColumnName("naam_placeholder");
 
+                    b.Property<string>("Reference")
+                        .HasColumnType("text")
+                        .HasColumnName("reference");
+
                     b.Property<string>("Status")
                         .IsRequired()
                         .HasColumnType("text")

services/projection-api/Projection.ReadModel/ProjectionDbContext.cs

@@ -24,6 +24,7 @@ public sealed class ProjectionDbContext(DbContextOptions<ProjectionDbContext> op
             e.HasKey(r => r.Id);
             e.Property(r => r.Id).HasColumnName("id");
             e.Property(r => r.Status).HasColumnName("status").IsRequired();
+            e.Property(r => r.Reference).HasColumnName("reference");
             e.Property(r => r.Bsn).HasColumnName("bsn");
             e.Property(r => r.NaamPlaceholder).HasColumnName("naam_placeholder");
         });
@@ -36,6 +37,7 @@ public sealed class ProjectionDbContext(DbContextOptions<ProjectionDbContext> op
             e.Property(r => r.Actie).HasColumnName("actie").IsRequired();
             e.Property(r => r.ZaakId).HasColumnName("zaak_id").IsRequired();
             e.Property(r => r.Resource).HasColumnName("resource").IsRequired();
+            e.Property(r => r.Reference).HasColumnName("reference");
             e.Property(r => r.ReceivedAt).HasColumnName("received_at");
         });
     }
@@ -46,6 +48,10 @@ public sealed class RegisterEntryRow
 {
     public required string Id { get; set; }
     public required string Status { get; set; }
+
+    /// <summary>The citizen-facing reference (the zaak's identificatie) — matches the submit confirmation (#78).</summary>
+    public string? Reference { get; set; }
+
     public string? Bsn { get; set; }
     public string? NaamPlaceholder { get; set; }
 }
@@ -61,5 +67,8 @@ public sealed class ProcessedNotificationRow
     /// the right status without reading OpenZaak (S-09b).</summary>
     public required string Resource { get; set; }
 
+    /// <summary>The zaak reference (identificatie), retained so a rebuild reprojects it without the ACL (#78).</summary>
+    public string? Reference { get; set; }
+
     public DateTimeOffset ReceivedAt { get; set; }
 }

services/projection-api/ProjectionApi.Api/Program.cs

@@ -23,7 +23,7 @@ app.MapGet("/register", async (ProjectionDbContext db, CancellationToken ct) =>
 {
     var rows = await db.RegisterEntries
         .OrderBy(r => r.Id)
-        .Select(r => new RegisterEntryDto(r.Id, r.Status, r.Bsn, r.NaamPlaceholder))
+        .Select(r => new RegisterEntryDto(r.Id, r.Status, r.Reference, r.Bsn, r.NaamPlaceholder))
         .ToListAsync(ct);
     return Results.Ok(rows);
 });
@@ -32,13 +32,13 @@ app.MapGet("/register/{id}", async (string id, ProjectionDbContext db, Cancellat
 {
     var row = await db.RegisterEntries
         .Where(r => r.Id == id)
-        .Select(r => new RegisterEntryDto(r.Id, r.Status, r.Bsn, r.NaamPlaceholder))
+        .Select(r => new RegisterEntryDto(r.Id, r.Status, r.Reference, r.Bsn, r.NaamPlaceholder))
         .SingleOrDefaultAsync(ct);
     return row is null ? Results.NotFound() : Results.Ok(row);
 });
 
 await app.RunAsync();
 
-public sealed record RegisterEntryDto(string Id, string Status, string? Bsn, string? NaamPlaceholder);
+public sealed record RegisterEntryDto(string Id, string Status, string? Reference, string? Bsn, string? NaamPlaceholder);
 
 public partial class Program;

tests/acceptance/Steps/RegisterProjectieBijwerkenSteps.cs

@@ -17,7 +17,8 @@ public sealed class RegisterProjectieBijwerkenSteps
     private readonly NotificationProjector _projector;
     private Notification? _notification;
 
-    public RegisterProjectieBijwerkenSteps() => _projector = new NotificationProjector(_log, _store);
+    public RegisterProjectieBijwerkenSteps()
+        => _projector = new NotificationProjector(_log, _store, new InMemoryAclReferenceClient());
 
     [Given("a zaak is created in OpenZaak with id \"(.*)\"")]
     public void GivenAZaakIsCreatedInOpenZaakWithId(string id)

tests/acceptance/Support/InMemoryProjectionStores.cs

@@ -38,3 +38,11 @@ public sealed class InMemoryProjectionStore : IProjectionStore
     public IReadOnlyList<RegisterEntry> RowsFor(string id)
         => [.. _byId.Values.Where(e => e.Id == id)];
 }
+
+/// <summary>A fake ACL client for the projection acceptance scenario: returns a reference derived
+/// from the zaak, so the projector can enrich rows without a running ACL (#78).</summary>
+public sealed class InMemoryAclReferenceClient : IAclClient
+{
+    public Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default)
+        => Task.FromResult("REG-" + zaakUrl.Segments[^1].Trim('/'));
+}