From 6fe2312ea9e6bda8c0e17f2e528ca7a1c330ba70 Mon Sep 17 00:00:00 2001
From: Stephen Davies <steve9164@gmail.com>
Date: Fri, 15 Mar 2024 01:38:16 +1100
Subject: [PATCH] Fix exception from objectArrayTrait using MergeStrategy
 topStratum

---
 CHANGES.md                                |  1 +
 lib/Traits/Decorators/objectArrayTrait.ts | 18 ++++++++++--------
 test/Traits/objectArrayTraitSpec.ts       |  6 ++++++
 3 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index ba852ff241..fd69dd5882 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,6 +2,7 @@
 
 #### next release (8.6.2)
 
+- Fixed exception thrown from `objectArrayTrait` when a model has 0 strata and a `MergeStrategy` of `topStratum`.
 - [The next improvement]
 
 #### 8.6.1 - 2024-03-14
diff --git a/lib/Traits/Decorators/objectArrayTrait.ts b/lib/Traits/Decorators/objectArrayTrait.ts
index fd581b77b0..947da7c213 100644
--- a/lib/Traits/Decorators/objectArrayTrait.ts
+++ b/lib/Traits/Decorators/objectArrayTrait.ts
@@ -156,14 +156,16 @@ export class ObjectArrayTrait<T extends ModelTraits> extends Trait {
     // If merge strategy is topStratum, then we only want to keep the ids that exist in the top stratum
     if (this.merge === MergeStrategy.TopStratum) {
       const topStratum = model.strataTopToBottom.values().next().value;
-
-      const topIds = this.getIdsAcrossStrata(new Map([["top", topStratum]]));
-      // Remove ids that don't exist in the top stratum
-      idsInCorrectOrder.forEach((id) => {
-        if (!topIds.has(id)) {
-          idsWithCorrectRemovals.delete(id);
-        }
-      });
+      // topStratum will be undefined if a model has 0 strata
+      if (topStratum !== undefined) {
+        const topIds = this.getIdsAcrossStrata(new Map([["top", topStratum]]));
+        // Remove ids that don't exist in the top stratum
+        idsInCorrectOrder.forEach((id) => {
+          if (!topIds.has(id)) {
+            idsWithCorrectRemovals.delete(id);
+          }
+        });
+      }
     }
 
     // Correct ids are:
diff --git a/test/Traits/objectArrayTraitSpec.ts b/test/Traits/objectArrayTraitSpec.ts
index bae94c227a..8efe0b7f05 100644
--- a/test/Traits/objectArrayTraitSpec.ts
+++ b/test/Traits/objectArrayTraitSpec.ts
@@ -392,6 +392,12 @@ describe("objectArrayTrait", function () {
     expect(model.inner[0].baz).toBe(true);
   });
 
+  it("no error when a model has 0 strata and merge = top", function () {
+    const terria = new Terria();
+    const model = new TestModelMergeTop("test", terria);
+    expect(model.inner.length).toBe(0);
+  });
+
   it("updates to reflect new strata added after evaluation (with merge = top)", function () {
     const terria = new Terria();
     const model = new TestModelMergeTop("test", terria);