# mutableSetValue(forKey:) Returns a mutable set proxy that provides read-write access to the unordered to-many relationship specified by a given key. ``` func mutableSetValue(forKey key: String) -> NSMutableSet ``` ## Parameters `key` The name of an unordered to-many relationship. ## Return Value A mutable set that provides read-write access to the unordered to-many relationship specified by `key`. ## Discussion Objects added to the mutable set proxy become related to the receiver, and objects removed from the mutable set become unrelated. The default implementation recognizes the same simple accessor methods and set accessor methods as [`value(forKey:)`](/documentation/ObjectiveC/NSObject-swift.class/value(forKey:)), and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that [`value(forKey:)`](/documentation/ObjectiveC/NSObject-swift.class/value(forKey:)) would return. The search pattern that `mutableSetValueForKey:` uses is described in [Accessor Search Patterns](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/KeyValueCoding/SearchImplementation.html#//apple_ref/doc/uid/20000955) in [Key-Value Coding Programming Guide](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple_ref/doc/uid/10000107i). --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)