Agile Web Development with Rails 5, 1st Edition

Participating in the Monitoring Process

Active Record controls the life cycle of model objects—it creates them, monitors them as they are modified, saves and updates them, and watches sadly as they are destroyed. Using callbacks, Active Record lets our code participate in this monitoring process. We can write code that gets invoked at any significant event in the life of an object. With these callbacks we can perform complex validation, map column values as they pass in and out of the database, and even prevent certain operations from completing.

Active Record defines sixteen callbacks. Fourteen of these form before/after pairs and bracket some operation on an Active Record object. For example, the before_destroy callback will be invoked just ...