Active Record implements the Active Record design pattern by Martin Fowler in Patterns of Enterprise Application Architecture (Addison Wesley). It's effectively a wrapper around a database table, with domain logic built into the wrapper. The Rails implementation adds two important innovations: you can do inheritance and manage relationships. These are some of the major features.
Active Record automatically adds properties, with accessors, to model objects. It also adds methods for simple CRUD database methods automatically. For example, in the view you just wrote, the view accesses the name property in trail, though the root model was empty:
class Trail < ActiveRecord::Base end
Rails uses methods to add methods that manage associations, automatically. You saw this example where a location has many trails:
class Location < ActiveRecord::Base has_many :trails end
As you have seen, has_many is a method, and :trails is a symbol, in this case, for the Ruby class trails.
You can use Active Record to compose objects from multiple tables, like this:
class Location < ActiveRecord::Base composed_of :street, :class_name => "Street", :mapping => %w(street name) end
Inheritance works, putting all subclasses in a single table with the parents:
class Product < ActiveRecord::Base end class Bike < Product end
These are some of the other features you can use. You can build recursive relationships, like trees. You can use Active Record to validate certain types of rules (for instance, there must be an existing location for a new trail). Active Record can notify an email address when some significant event happens.
Active Record also has good plumbing. It supports transactions and error logging. You can look at the metadata for the columns for a table, and support multiple database types. It also provides support that makes it easy for you to build test fixtures. Active Record is a powerful framework and a credible competitor to Java's ORM frameworks.
Action Pack deals with requests in two parts: the controller and the view. Requests come into Action Pack through a dispatcher. The dispatcher routes the request to a controller, which invokes any model logic and sends the request to a template-driven view system. The template engine fires the Ruby template, which may execute Ruby code, and returns the resulting HTML to the browser. The flow, shown in Figure 7-3, is reminiscent of Struts. There are a few differences. For example, the controller has a group of actions, instead of encapsulating each action in a different class. If you wanted to refactor, you'd let actions share methods.
Figure 7-3. Ruby on Rails is actually made up of several existing frameworks, most notably Active Record and Action Pack
The Action Pack splits the request into a controller part and a view part. With Rails, a whole lot happens automatically. In some ways, that's bad. You can't see all the methods or the attributes on your class, and you don't even know what they are unless you look at the database. In other ways, it's a highly productive way to work. You can change your model, schema, and view in many cases just by adding columns to the schema. Let's take a fuller look at the capabilities of Action Pack.
Action Pack goes beyond simple request processing. It contains many capabilities that make it easier to develop web applications. I'll touch on some of the major capabilities here.
As you've seen, Action Pack uses Ruby as the scripting language. Java developers frown on embedding Java into a JSP, but I'd suggest that code will be in the view regardless of whether it's in Ruby. Early on, some vocal zealots overreacted to the early proliferation of Java scriptlets and decreed that MVC means "no code on the page." Many Ruby developers believe that code that is concerned with the view (and only the view) does belong on the page. Burying Java code in a custom tag only complicates and confuses the issue.
Ruby provides a far friendlier scripting language than JSTL tags, for example. Like servlets, Action Pack lets you attach filters for things like authentication. Action Pack also handles some convenience design elements, like automatically paginating your result sets and providing navigation links.
Action Pack also has some features that make it easier to build components, like helper classes (to render a date, for example), a layout sharing feature (similar to Tiles, if you're familiar with Struts), intracomponent communication, and pretty good Ajax integration. Like Struts and Spring, Action Pack provides good support for building and validating forms.
You'll need to manage your solution, and Action Pack builds in some features to help. It enables logging, caching at three levels (page, action, and fragment), and benchmarking support. Developers can use integrated support for unit testing and debugging. It's not as powerful as Struts in some ways, but it's much simpler, and highly customizable.