diff --git a/functions/advance_rotation/handler.ts b/functions/advance_rotation/handler.ts
index 9dc5ecc..9bff74b 100644
--- a/functions/advance_rotation/handler.ts
+++ b/functions/advance_rotation/handler.ts
@@ -1,6 +1,4 @@
 import { SlackFunction } from "deno-slack-sdk/mod.ts";
-import { DatastoreItem } from "deno-slack-api/types.ts";
-
 import { AdvanceRotationFunctionDefinition } from "./definition.ts";
 
 import RotationDatastore from "../../datastores/rotations.ts";
@@ -12,10 +10,7 @@ export default SlackFunction(
     // TODO: the `rotation` input is typed as any - what can we do to improve this?
     const { order: assigneeOrder } = inputs.rotation;
     const {
-      rotation_trigger_id,
       channel,
-      message,
-      start_time,
       repeats_every,
       repeats_every_number,
       last_advance_time,
@@ -26,29 +21,21 @@ export default SlackFunction(
     const first = assigneeOrder.shift();
     const assignees = [...assigneeOrder, first];
 
-    // 2: generate a new rotation object
-    const newRotation: DatastoreItem<typeof RotationDatastore.definition> = {
-      rotation_trigger_id,
-      channel,
-      message,
-      start_time,
-      repeats_every,
-      repeats_every_number,
-      order: assignees,
-      last_advance_time: next_advance_time,
-      next_advance_time: getNextAdvanceTimeInSec(
-        last_advance_time,
-        repeats_every_number,
-        repeats_every,
-      ),
-    };
-
-    // 3: update or create the rotation in the datastore
-    const putResponse = await client.apps.datastore.put<
+    // 2: update the rotation in the datastore
+    const putResponse = await client.apps.datastore.update<
       typeof RotationDatastore.definition
     >({
       datastore: RotationDatastore.name,
-      item: newRotation,
+      item: {
+        channel,
+        order: assignees,
+        last_advance_time: next_advance_time,
+        next_advance_time: getNextAdvanceTimeInSec(
+          last_advance_time,
+          repeats_every_number,
+          repeats_every,
+        ),
+      },
     });
 
     if (!putResponse.ok) {
diff --git a/functions/advance_rotation/handler_test.ts b/functions/advance_rotation/handler_test.ts
index 252cbd3..773a648 100644
--- a/functions/advance_rotation/handler_test.ts
+++ b/functions/advance_rotation/handler_test.ts
@@ -11,9 +11,15 @@ const { createContext } = SlackFunctionTester(
 // Replaces globalThis.fetch with the mocked copy
 mf.install();
 
-mf.mock("POST@/api/apps.datastore.put", () => {
+mf.mock("POST@/api/apps.datastore.update", async (args) => {
+  const body = await args.formData();
+  const item = body.get("item") ?? "{}";
+  const { order } = JSON.parse(item.toString());
+
   return new Response(
-    `{"ok": true, "item": {"repeats_every": "day"}}`,
+    `{"ok": true, "item": {"repeats_every": "day", "order": ${
+      JSON.stringify(order)
+    } }}`,
     {
       status: 200,
     },
@@ -23,7 +29,7 @@ mf.mock("POST@/api/apps.datastore.put", () => {
 Deno.test("Sample function test", async () => {
   const inputs = {
     rotation: {
-      order: ["testuser1"],
+      order: ["testuser1", "realuser2", "fakeuser3"],
       channel: "channelid",
       message: "testmessage",
       start_time: 1,
@@ -35,8 +41,10 @@ Deno.test("Sample function test", async () => {
   };
 
   const { outputs } = await AdvanceRotationFunction(createContext({ inputs }));
-  await assertEquals(
-    outputs?.rotation.repeats_every,
-    "day",
-  );
+  assertEquals(outputs?.rotation.repeats_every, "day");
+  assertEquals(outputs?.rotation.order, [
+    "realuser2",
+    "fakeuser3",
+    "testuser1",
+  ]);
 });