diff --git a/CHANGELOG.md b/CHANGELOG.md
index 91b7ab6..65cfc80 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,7 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
+## [2.9.0] - 2024-09-10
+### Added
+- `processValues()` now allows setting the ID of an existing row on the returned object, in which
+case a new row will not be inserted, and the specified ID will be returned in the list of row IDs.
 
 ## [2.8.0] - 2023-12-22
 ### Added
@@ -151,7 +154,7 @@ return early if passed an empty IDs array.
 ### Changed
 - Initial stable release
 
-[Unreleased]: https://github.com/theodorejb/phaster/compare/v2.8.0...HEAD
+[2.9.0]: https://github.com/theodorejb/phaster/compare/v2.8.0...v2.9.0
 [2.8.0]: https://github.com/theodorejb/phaster/compare/v2.7.0...v2.8.0
 [2.7.0]: https://github.com/theodorejb/phaster/compare/v2.6.0...v2.7.0
 [2.6.0]: https://github.com/theodorejb/phaster/compare/v2.5.0...v2.6.0
diff --git a/src/Entities.php b/src/Entities.php
index 31cda30..9f2f792 100644
--- a/src/Entities.php
+++ b/src/Entities.php
@@ -263,15 +263,32 @@ public function addEntities(array $entities): array
 
         $defaultValues = $this->getDefaultValues();
         $rows = [];
-
-        foreach ($entities as $entity) {
-            $data = array_replace_recursive($defaultValues, $entity);
-            $row = Helpers::allPropertiesToColumns($this->map, $this->processValues($data, []));
-            $rows[] = $this->processRow($row, []);
+        $existingIds = [];
+
+        foreach ($entities as $key => $entity) {
+            unset($entity[$this->idField]); // any ID posted to API should be ignored
+            $entity = array_replace_recursive($defaultValues, $entity);
+            $entity = $this->processValues($entity, []);
+
+            // if processValues sets an ID for an existing item, don't insert a new row for it
+            if (isset($entity[$this->idField])) {
+                $id = $entity[$this->idField];
+                if (!is_int($id)) {
+                    throw new \Exception('ID value set by processValues must be an integer');
+                }
+                $existingIds[$key] = $id;
+            } else {
+                $row = Helpers::allPropertiesToColumns($this->map, $entity);
+                $rows[] = $this->processRow($row, []);
+            }
         }
 
         try {
-            return $this->db->insertRows($this->getTableName(), $rows, $this->getIdentityIncrement())->getIds();
+            $ids = $this->db->insertRows($this->getTableName(), $rows, $this->getIdentityIncrement())->getIds();
+            foreach ($existingIds as $offset => $id) {
+                array_splice($ids, $offset, 0, [$id]);
+            }
+            return $ids;
         } catch (SqlException $e) {
             throw $this->properException($e);
         }
diff --git a/test/EntitiesDbTest.php b/test/EntitiesDbTest.php
index ff3da39..cd8afe4 100644
--- a/test/EntitiesDbTest.php
+++ b/test/EntitiesDbTest.php
@@ -362,6 +362,8 @@ public function testModernUsers(Entities $entities, PeachySql $db): void
         }
 
         $ids = $entities->addEntities($users);
+        $this->assertSame(-42, $ids[1]); // manually set ID in processValues
+
         $db->insertRow('UserThings', ['user_id' => $ids[3]]);
 
         $actual = $entities->getEntitiesByIds([$ids[2], $ids[3]], ['id', 'name', 'isDisabled', 'computed', 'weight', 'thing.uid']);
diff --git a/test/src/ModernUsers.php b/test/src/ModernUsers.php
index 3e82ace..6889330 100644
--- a/test/src/ModernUsers.php
+++ b/test/src/ModernUsers.php
@@ -59,8 +59,12 @@ protected function getBaseQuery(QueryOptions $options): string
 
     protected function processValues(array $data, array $ids): array
     {
-        if (count($ids) === 0 && $data['name'] === 'Modern user 3') {
-            $data['name'] = 'Modern user 3 modified';
+        if (count($ids) === 0) {
+            if ($data['name'] === 'Modern user 3') {
+                $data['name'] = 'Modern user 3 modified';
+            } elseif ($data['name'] === 'Modern user 2') {
+                $data['id'] = -42; // don't insert row for this item
+            }
         }
 
         return $data;