# NSDerivedAttributeDescription A description of an attribute that derives its value by performing a calculation on a related attribute. ``` class NSDerivedAttributeDescription ``` ## Overview Use derived attributes to optimize fetch performance; for example: - Create a derived `searchName` attribute to reflect a `name` attribute with case and diacritics removed for more efficient comparison. - Create a derived `relationshipCount` attribute to reflect the number of objects in a relationship and avoid having to do a join. Derived attributes support the following expressions: |**Expression** |**Description** |**Example** | |-------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------| |to-one keypath |A single value to replicate. |`name` or `author.name`| |to-one keypath with a function |The result of calling a function on a single value.  Supported functions include `canonical:`, `uppercase:`, and `lowercase:`.  The `canonical:` function returns a case- and diacritic-insensitive String value.|`canonical:(name)` | |to-many keypath with a function|The result of calling an aggregate function on a set of values.  Supported functions include `@count` and `@sum`. |`friends.@count` | |time |The current time. |`now()` | > Important: > Data recomputes derived attributes when you save a context. A managed object’s property does not reflect unsaved changes until you save the context and refresh the object. ## Topics ### Specifying the Derivation Expression [`derivationExpression`](/documentation/CoreData/NSDerivedAttributeDescription/derivationExpression) An expression for generating derived data. --- 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)