Rails: Up and Running, 2nd Edition

What You Haven’t Seen

This implementation of categories is like a filesystem. Each directory has a set of files, just as each of our categories has a set of photos. Another macro, called acts_as_nested_set, would be useful if you wanted to find all of the folders in the All tree. Active Record is too big to cover in detail in such a short book, but we’ll point you to a few things that you should know:

Event callbacks

Active Record can call a method when an event happens, such as before a save, after creation, or when a model changes.

Reflection

Active Record will let you find attributes (as you’ve seen), and will also find all of the associations, or relationships, defined on a model.

Versioning

Active Record uses the column lock_version, if it exists, to manage concurrency using a technique called optimistic locking. With this technique, a database engine can store multiple versions of each piece of data and maintain database integrity if many applications need the same piece of data.

Count caching

Rather than using SQL to compute the number of certain types of objects, Active Record can cache the number of items in a collection for performance.

Polymorphic associations

You can design a model that has belongs_to relationships to more than one model. For example, an address can belong to a person and a shipment.

Enhancements

Active Record gets enhanced often. We recommend that you periodically check the documentation and watch the various Rails mailing lists if you’re going to be doing regular ...