RailsA Peek under the covers

Brian Sam-Bodden

Rails

A framework...

for building database-backed web applications

that at first wow us with speed

Rails can be seen as a collection of...

mini-languages

for implementing MVC

on the web

then came the headlines...

and we all wondered...

what am I doing wrong?

too much

the answer

and we reacted

the way we knew better

...with frameworks

Lessons of Rails

Integrated

Familiar idioms on every tier

Seamless front to back stack

Avoiding manually defining or gluing the tiers

CoC

“Flexibility is overrated, Constraints are liberating”

Convention Over Configuration

DRYDo it once, do it right and do it in the right place

Tight feedback loop

We want the world and we want it NOW!

Instant Change

Getting advanced behavior with very little code

Without sacrificing readability

Terse Expressiveness

Did the clones catch up?

Yes and Not quite, Sort of

✓CoC

✓ Instant Change๏Terse Expressiveness

✓DRY

✓Integrated

What’s Missing?

Rubyis...

✓Object-Oriented

✓Reflective ✓Dynamic

✓ Interpreted

✓General Purpose

✓ Multi-paradigm

✓ Garbage-collected

✓Elegant

Without Ruby there would be no Rails

Ruby’s Dynamism is the key ingredient to the Rails framework

Open Classes

Say we have an Array like:

[1,2,3,4,5]and we want to sum of all of its elements

Let’s try something like:

[1,2,3,4,5].sum

Doh! Doesn’t work in Ruby … yet!

Ruby classes are open

We can teach the Array class a new behavior

Let’s try again!

Load the file containing the enhancements

Since Ruby classes are open...

You can statically enhance ANY class

Code as Data

When code can be manipulated as dataa whole world of possibilities opens

In Ruby you can evaluate the contents of a string

Code that spits out code

Ruby provides the eval family of methods for runtime execution of code stored in strings

eval will evaluate any string

instance_eval can evaluate a string or a code block

in the context of the receiver

class_eval can evaluate a string or a code block

in the context of the class or module it is called on

Meta-programming

...is about programs that write programs

Meta-programming

...it’s a superb tool for building frameworks

...it’s the key ingredient for building domain-specific languages

Ruby is a great language for meta-programming because

...is dynamic and reflexive

...open and malleable

...clean unencumbered syntax

...code is data, data is code

...programming event model

...uses the Ruby meta-programming techniques to accomplish most of its “magic”

...uses meta-programming to bring the language closer to the problem at hand

Rails

...is a domain-specific language for building web applications

Singleton Class

All objects are open to modification

You can change a particular instance of a class

Ruby uses a proxy class known as the singleton class

Meta-class: The singleton for Class objects

With access to a class object’s meta-class we can then use Ruby meta-programming techniques to

enhance the class

Rails relies heavily on Ruby’s ability to dynamically enhance a class

The previous example could have also been accomplished using the define_method

Accessing the singleton meta-class explicitly

Inheritance the Ruby Way

Rails uses this technique in ActiveRecord

Ruby Meta-programming techniques can be used to write DSLs that use class methods to enhance

subclasses

Using the meta-class (singleton class) of the subclasses

Let’s create a module with a “Base” class

Methods get added to our classes with simple declarationsCan you say DSL!

Rails uses this technique in ActiveRecord

Now our class has a new set of class methods

AOP the Ruby Way

With alias_method you can wrap an existing method

Meta-Programming Events

Included Hook

Ruby has a rich event model associated with static and dynamic changes of the code

Method Missing

Ruby provides several hook methods that can be used to created custom behaviors

method_missing

method_added

method_removed

Let’s implement the greeter example using method_missing

There is more to Ruby than Rails!

Building a Framework? Give Ruby a try!

Build the language up towards the problem

Meta-programming is no longer just for the Lisp folks

The future? Powerful Java APIs exposed with Ruby DSLs

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