Pundit
Minimal authorization through OO design and pure Ruby classes
...It centers around policy classes—one policy per model or resource—that define what actions a given user is permitted to perform (e.g., show?, update?, destroy?). Each policy method returns a boolean, and can be structured using combinators or shared logic to keep things DRY. In controllers and views, Pundit provides helpers like authorize, policy_scope, and policy to enforce those rules cleanly and consistently. The policy_scope feature is especially helpful—it restricts index or list queries to only the records the current user can see by applying scopes defined in the policy class. Pundit encourages placing authorization logic close to the model domain without muddling controllers or views, making permissions easier to reason about and testable.