Add some tests for this new version doesnt follow id

src/documents/tests/test_api_document_versions.py

@@ -827,6 +827,67 @@ class TestDocumentVersioningApi(DirectoriesMixin, APITestCase):
         self.assertEqual(resp.status_code, status.HTTP_200_OK)
         self.assertEqual(resp.data["content"], "v1-content")
 
+    def _make_root_with_out_of_order_versions(self) -> tuple[Document, ...]:
+        """
+        A root whose newest version has a *lower* id than an older one, which is
+        what merging an existing document in as a version produces.
+        """
+        root = Document.objects.create(
+            title="root",
+            checksum="root",
+            mime_type="application/pdf",
+            content="root-content",
+        )
+        newest = Document.objects.create(
+            title="newest",
+            checksum="newest",
+            mime_type="application/pdf",
+            content="newest-content",
+        )
+        older = Document.objects.create(
+            title="older",
+            checksum="older",
+            mime_type="application/pdf",
+            root_document=root,
+            version_index=1,
+            content="older-content",
+        )
+        # Assigned last, so `newest` has the lower id despite being the later version
+        newest.root_document = root
+        newest.version_index = 2
+        newest.save()
+        return root, newest, older
+
+    def test_retrieve_uses_version_index_not_id_for_latest(self) -> None:
+        root, _, _ = self._make_root_with_out_of_order_versions()
+
+        resp = self.client.get(f"/api/documents/{root.id}/")
+
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(resp.data["content"], "newest-content")
+
+    def test_list_uses_version_index_not_id_for_latest(self) -> None:
+        self._make_root_with_out_of_order_versions()
+
+        resp = self.client.get("/api/documents/?fields=id,content")
+
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(
+            [doc["content"] for doc in resp.data["results"]],
+            ["newest-content"],
+        )
+
+    def test_versions_are_listed_newest_first_with_root_last(self) -> None:
+        root, newest, older = self._make_root_with_out_of_order_versions()
+
+        resp = self.client.get(f"/api/documents/{root.id}/")
+
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(
+            [(version["id"], version["is_root"]) for version in resp.data["versions"]],
+            [(newest.id, False), (older.id, False), (root.id, True)],
+        )
+
 
 class TestVersionAwareFilters(TestCase):
     def test_title_content_filter_falls_back_to_content(self) -> None: