fix(update): correctly apply timestamps to update pipelines

Fix #11151

lib/helpers/update/applyTimestampsToUpdate.js

@@ -44,7 +44,7 @@ function applyTimestampsToUpdate(now, createdAt, updatedAt, currentUpdate, optio
 
   if (Array.isArray(updates)) {
     // Update with aggregation pipeline
-    updates.push({ $set: { updatedAt: now } });
+    updates.push({ $set: { [updatedAt]: now } });
 
     return updates;
   }

test/helpers/update.applyTimestampsToUpdate.test.js

@@ -0,0 +1,15 @@
+'use strict';
+
+const applyTimestampsToUpdate = require('../../lib/helpers/update/applyTimestampsToUpdate');
+const assert = require('assert');
+
+describe('applyTimestampsToUpdate', function() {
+  it('handles update pipelines (gh-11151)', function() {
+    const update = [{ $set: { title: 'mongoose-with-aggregate-set' } }];
+    const now = new Date('2016-01-01');
+    const res = applyTimestampsToUpdate(now, 'created_at', 'updated_at', update, {});
+
+    assert.equal(res.length, 2);
+    assert.equal(res[1].$set.updated_at.valueOf(), now.valueOf());
+  });
+});