test(api): cover the CRE.id tie-break in _root_cres_query's ordering

Same gap as the all_cres test on #1060: every ordering fixture used a
distinct external_id, so the test passed whether or not the sort carried
its CRE.id half. external_id is not unique on its own -- the constraint is
on (name, external_id) -- so ties are possible and the tie-break is what
makes the order total.

Two root CREs share an external_id here and are inserted in the opposite
order to their primary keys, which are fixed rather than generated.
Dropping CRE.id from the sort now fails the test.

Author: kunalKumar-13

application/tests/web_main_test.py

@@ -625,6 +625,33 @@ def test_find_root_cres_pagination_integration(self) -> None:
             )
             self.assertEqual(404, response.status_code)
 
+    def test_find_root_cres_orders_duplicate_external_ids_by_primary_key(self) -> None:
+        """external_id is not unique on its own: the constraint is on
+        (name, external_id), so two root CREs may carry the same one. Ordering
+        by external_id alone would leave those ties undefined, which is what the
+        CRE.id half of _root_cres_query's sort is for. The primary keys are
+        fixed here rather than generated, so the expected order cannot drift."""
+        collection = db.Node_collection().with_graph()
+        # Inserted in the opposite order to their primary keys.
+        for pk, name in (
+            ("bbbbbbbb-0000-0000-0000-000000000000", "tie-second"),
+            ("aaaaaaaa-0000-0000-0000-000000000000", "tie-first"),
+        ):
+            collection.session.add(
+                db.CRE(id=pk, external_id="500-500", name=name, description="")
+            )
+        collection.session.commit()
+
+        with self.app.test_client() as client:
+            response = client.get(
+                "/rest/v1/root_cres?per_page=10&page=1",
+                headers={"Content-Type": "application/json"},
+            )
+            self.assertEqual(200, response.status_code)
+            names = [doc["name"] for doc in json.loads(response.data)["data"]]
+
+        self.assertEqual(["tie-first", "tie-second"], names)
+
     def test_find_root_cres_rejects_malformed_pagination(self) -> None:
         """A non-integer page or per_page is a client error, not a 500."""
         collection = db.Node_collection().with_graph()