From ce36fc6fcac2874c1ee125823668a678d7004416 Mon Sep 17 00:00:00 2001
From: vejrj <77059398+vejrj@users.noreply.github.com>
Date: Mon, 5 Jan 2026 13:36:26 +0100
Subject: [PATCH] mergeSchemaDefinitions implementation bug

---
 .../decodeASTSchema.test.ts.snap              |  2 +-
 .../__tests__/mergeSchemaDefinitions.test.ts  | 80 +++++++++++++++++++
 2 files changed, 81 insertions(+), 1 deletion(-)
 create mode 100644 packages/supermassive/src/utilities/__tests__/mergeSchemaDefinitions.test.ts

diff --git a/packages/supermassive/src/utilities/__tests__/__snapshots__/decodeASTSchema.test.ts.snap b/packages/supermassive/src/utilities/__tests__/__snapshots__/decodeASTSchema.test.ts.snap
index 7af3e63f2..d69d6c6fd 100644
--- a/packages/supermassive/src/utilities/__tests__/__snapshots__/decodeASTSchema.test.ts.snap
+++ b/packages/supermassive/src/utilities/__tests__/__snapshots__/decodeASTSchema.test.ts.snap
@@ -30,7 +30,7 @@ type AnnotatedObject {
 
 type UndefinedType
 
-interface Bar {
+interface Bar implements Two {
   one: Type
   four(argument: String = "string"): String
   two(argument: InputType!): Type
diff --git a/packages/supermassive/src/utilities/__tests__/mergeSchemaDefinitions.test.ts b/packages/supermassive/src/utilities/__tests__/mergeSchemaDefinitions.test.ts
new file mode 100644
index 000000000..93f643359
--- /dev/null
+++ b/packages/supermassive/src/utilities/__tests__/mergeSchemaDefinitions.test.ts
@@ -0,0 +1,80 @@
+import { parse } from "graphql";
+import { mergeSchemaDefinitions } from "../mergeSchemaDefinitions";
+import { encodeASTSchema } from "../encodeASTSchema";
+import { SchemaDefinitions } from "../../schema/definition";
+
+function schema(sdl: string): SchemaDefinitions[] {
+  const doc = parse(sdl);
+  return encodeASTSchema(doc);
+}
+
+describe("mergeSchemaDefinitions", () => {
+  it("should copy interfaces from source when target interface has none", () => {
+    const defs = schema(`
+      interface Entity {
+        id: ID
+      }
+      
+      interface Entity implements Node & Named {
+        name: String
+      }
+    `);
+    const result = mergeSchemaDefinitions({ types: {}, directives: [] }, defs);
+    expect(result).toMatchInlineSnapshot(`
+      {
+        "directives": [],
+        "types": {
+          "Entity": [
+            3,
+            {
+              "id": 5,
+              "name": 1,
+            },
+            [
+              "Node",
+              "Named",
+            ],
+          ],
+        },
+      }
+    `);
+  });
+
+  it("should copy interfaces from source when interface extension has none", () => {
+    const defs1 = schema(`
+      extend interface Entity {
+        id: ID
+      }
+    `);
+
+    const defs2 = schema(` 
+      interface Entity implements Node & Named {
+        name: String
+      }
+    `);
+    const mergeResult1 = mergeSchemaDefinitions(
+      { types: {}, directives: [] },
+      defs1,
+    );
+
+    const mergeResult2 = mergeSchemaDefinitions(mergeResult1, defs2);
+    expect(mergeResult2).toMatchInlineSnapshot(`
+      {
+        "directives": [],
+        "types": {
+          "Entity": [
+            3,
+            {
+              "id": 5,
+              "name": 1,
+            },
+            [
+              "Node",
+              "Named",
+            ],
+          ],
+        },
+      }
+    `);
+  });
+});