From c6e4c7820ad0cabcfb0537a5ee49a11a78c074f6 Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Thu, 22 Oct 2026 08:32:45 -0400
Subject: [PATCH] fix(configdb): InstanceLocator single untracked projected
 join (review 2026-10-21 08/P7)

---
 .../Services/InstanceLocator.cs                | 18 ++++++++----------
 .../RepositoryCoverageTests.cs                 | 18 ++++++++++++++++++
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Services/InstanceLocator.cs b/src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Services/InstanceLocator.cs
index 18b1eaea..ed0dbcf6 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Services/InstanceLocator.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Services/InstanceLocator.cs
@@ -22,15 +22,13 @@ public class InstanceLocator : IInstanceLocator
         string instanceUniqueName,
         CancellationToken cancellationToken = default)
     {
-        var instance = await _context.Set<Commons.Entities.Instances.Instance>()
-            .FirstOrDefaultAsync(i => i.UniqueName == instanceUniqueName, cancellationToken);
-
-        if (instance == null)
-            return null;
-
-        var site = await _context.Set<Commons.Entities.Sites.Site>()
-            .FindAsync(new object[] { instance.SiteId }, cancellationToken);
-
-        return site?.SiteIdentifier;
+        // One untracked projected join: this sits on the inbound-API Route.To hot path and
+        // only needs a single string, so it must not materialise (and track) two entities.
+        return await (
+                from i in _context.Set<Commons.Entities.Instances.Instance>().AsNoTracking()
+                join s in _context.Set<Commons.Entities.Sites.Site>().AsNoTracking() on i.SiteId equals s.Id
+                where i.UniqueName == instanceUniqueName
+                select s.SiteIdentifier)
+            .FirstOrDefaultAsync(cancellationToken);
     }
 }
diff --git a/tests/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs b/tests/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs
index 8efd568e..40915e3c 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs
@@ -1486,6 +1486,24 @@ public class InstanceLocatorTests : IDisposable
         Assert.Equal("SITE-001", result);
     }
 
+    /// <summary>The lookup must not track entities (single untracked projection, review 08/P7).</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task GetSiteIdForInstance_DoesNotTrackEntities()
+    {
+        var site = new Site("Site1", "SITE-001");
+        var template = new Template("T1");
+        _context.Sites.Add(site);
+        _context.Templates.Add(template);
+        await _context.SaveChangesAsync();
+        _context.Instances.Add(new Instance("Pump1") { SiteId = site.Id, TemplateId = template.Id });
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        Assert.Equal("SITE-001", await _locator.GetSiteIdForInstanceAsync("Pump1"));
+        Assert.Empty(_context.ChangeTracker.Entries());
+    }
+
     /// <summary>Verifies that getting the site id for a missing instance returns null.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]