From 0de465044a70adfe49f771b6b7976b1d841ac056 Mon Sep 17 00:00:00 2001
From: pawelblaszczyk5 <internal@pawelblaszczyk.dev>
Date: Wed, 13 Aug 2025 22:11:13 +0200
Subject: [PATCH 1/2] fix: make item drafts mutable

---
 .changeset/shiny-phones-stick.md |  5 +++
 packages/db/src/collection.ts    | 13 +++---
 packages/db/src/types.ts         | 74 ++++++++++++++++++++++++++++++++
 3 files changed, 87 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/shiny-phones-stick.md

diff --git a/.changeset/shiny-phones-stick.md b/.changeset/shiny-phones-stick.md
new file mode 100644
index 000000000..2ef8f9345
--- /dev/null
+++ b/.changeset/shiny-phones-stick.md
@@ -0,0 +1,5 @@
+---
+"@tanstack/db": patch
+---
+
+fix: make item drafts mutable
diff --git a/packages/db/src/collection.ts b/packages/db/src/collection.ts
index eea7905cd..131698ee3 100644
--- a/packages/db/src/collection.ts
+++ b/packages/db/src/collection.ts
@@ -58,6 +58,7 @@ import type {
   Transaction as TransactionType,
   TransactionWithMutations,
   UtilsRecord,
+  WritableDeep,
 } from "./types"
 import type { IndexOptions } from "./indexes/index-options.js"
 import type { BaseIndex, IndexResolver } from "./indexes/base-index.js"
@@ -1810,32 +1811,34 @@ export class CollectionImpl<
   // Overload 1: Update multiple items with a callback
   update<TItem extends object = T>(
     key: Array<TKey | unknown>,
-    callback: (drafts: Array<TItem>) => void
+    callback: (drafts: Array<WritableDeep<TItem>>) => void
   ): TransactionType
 
   // Overload 2: Update multiple items with config and a callback
   update<TItem extends object = T>(
     keys: Array<TKey | unknown>,
     config: OperationConfig,
-    callback: (drafts: Array<TItem>) => void
+    callback: (drafts: Array<WritableDeep<TItem>>) => void
   ): TransactionType
 
   // Overload 3: Update a single item with a callback
   update<TItem extends object = T>(
     id: TKey | unknown,
-    callback: (draft: TItem) => void
+    callback: (draft: WritableDeep<TItem>) => void
   ): TransactionType
 
   // Overload 4: Update a single item with config and a callback
   update<TItem extends object = T>(
     id: TKey | unknown,
     config: OperationConfig,
-    callback: (draft: TItem) => void
+    callback: (draft: WritableDeep<TItem>) => void
   ): TransactionType
 
   update<TItem extends object = T>(
     keys: (TKey | unknown) | Array<TKey | unknown>,
-    configOrCallback: ((draft: TItem | Array<TItem>) => void) | OperationConfig,
+    configOrCallback:
+      | ((draft: WritableDeep<TItem> | Array<WritableDeep<TItem>>) => void)
+      | OperationConfig,
     maybeCallback?: (draft: TItem | Array<TItem>) => void
   ) {
     if (typeof keys === `undefined`) {
diff --git a/packages/db/src/types.ts b/packages/db/src/types.ts
index 28b25e0ef..3c9da85f8 100644
--- a/packages/db/src/types.ts
+++ b/packages/db/src/types.ts
@@ -628,3 +628,77 @@ export type ChangeListener<
   T extends object = Record<string, unknown>,
   TKey extends string | number = string | number,
 > = (changes: Array<ChangeMessage<T, TKey>>) => void
+
+// Adapted from https://github.com/sindresorhus/type-fest
+// MIT License Copyright (c) Sindre Sorhus
+
+type BuiltIns =
+  | null
+  | undefined
+  | string
+  | number
+  | boolean
+  | symbol
+  | bigint
+  | void
+  | Date
+  | RegExp
+
+type HasMultipleCallSignatures<
+  T extends (...arguments_: Array<any>) => unknown,
+> = T extends {
+  (...arguments_: infer A): unknown
+  (...arguments_: infer B): unknown
+}
+  ? B extends A
+    ? A extends B
+      ? false
+      : true
+    : true
+  : false
+
+type WritableMapDeep<MapType extends ReadonlyMap<unknown, unknown>> =
+  MapType extends ReadonlyMap<infer KeyType, infer ValueType>
+    ? Map<WritableDeep<KeyType>, WritableDeep<ValueType>>
+    : MapType
+
+type WritableSetDeep<SetType extends ReadonlySet<unknown>> =
+  SetType extends ReadonlySet<infer ItemType>
+    ? Set<WritableDeep<ItemType>>
+    : SetType
+
+type WritableObjectDeep<ObjectType extends object> = {
+  -readonly [KeyType in keyof ObjectType]: WritableDeep<ObjectType[KeyType]>
+}
+
+type WritableArrayDeep<ArrayType extends ReadonlyArray<unknown>> =
+  ArrayType extends readonly []
+    ? []
+    : ArrayType extends readonly [...infer U, infer V]
+      ? [...WritableArrayDeep<U>, WritableDeep<V>]
+      : ArrayType extends readonly [infer U, ...infer V]
+        ? [WritableDeep<U>, ...WritableArrayDeep<V>]
+        : ArrayType extends ReadonlyArray<infer U>
+          ? Array<WritableDeep<U>>
+          : ArrayType extends Array<infer U>
+            ? Array<WritableDeep<U>>
+            : ArrayType
+
+export type WritableDeep<T> = T extends BuiltIns
+  ? T
+  : T extends (...arguments_: Array<any>) => unknown
+    ? {} extends WritableObjectDeep<T>
+      ? T
+      : HasMultipleCallSignatures<T> extends true
+        ? T
+        : ((...arguments_: Parameters<T>) => ReturnType<T>) &
+            WritableObjectDeep<T>
+    : T extends ReadonlyMap<unknown, unknown>
+      ? WritableMapDeep<T>
+      : T extends ReadonlySet<unknown>
+        ? WritableSetDeep<T>
+        : T extends ReadonlyArray<unknown>
+          ? WritableArrayDeep<T>
+          : T extends object
+            ? WritableObjectDeep<T>
+            : unknown

From c356557ff8190403f044d9b5889757ad72ffde22 Mon Sep 17 00:00:00 2001
From: Sam Willis <sam.willis@gmail.com>
Date: Mon, 25 Aug 2025 12:26:26 +0100
Subject: [PATCH 2/2] update changeset

---
 .changeset/shiny-phones-stick.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/shiny-phones-stick.md b/.changeset/shiny-phones-stick.md
index 2ef8f9345..33e6c3988 100644
--- a/.changeset/shiny-phones-stick.md
+++ b/.changeset/shiny-phones-stick.md
@@ -2,4 +2,4 @@
 "@tanstack/db": patch
 ---
 
-fix: make item drafts mutable
+mark item drafts as a `mutable` type