Ivan Nemytchenko's proposed layer between services and Active Record. A mutator is a plain procedure (module or class of functions) handling a single purpose — creating, editing, or deleting records — as an atomic operation so callers never see half-baked entities with unfilled associations. Services call mutators to persist results instead of touching Active Record directly, keeping services focused on business logic and external interactions. Mutators can absorb counters/denormalization code previously expressed via callbacks, and effectively replace callbacks entirely by making the full creation procedure explicit. Complements the 'lazy' progression: simple cases stay in the controller; medium cases add a mutator; complex cases add a service that orchestrates mutators, managers, and jobs.