miceda

An Example

So, you’re building a Web application that allows users to customize a car before they purchase it.

Here’s what the current car table looks like ( in Migration speak ):

  create_table "cars" do |t|
    t.column "make",  :string
    t.column "model", :string
    t.column "new_smell", :boolean
  end
  

The tasking…

You boss comes in and says “Hey! I got a great idea! Let’s give users the ability to manipulate/change the color of the car…” Immediately your brain snaps into design mode. Hmmm…each car may have many color choices…do I need to add another table just for colors? You ask your boss, “Do we care what colors people are choosing? For example, will we ever want to ask the question how many people chose a Red car”? “No”, your Boss replies, “keep it simple and have it done yesterday!”

ActiveRecord to the rescue

Ok, I like keeping it simple. I’m just going to stuff it all into the car table. Fortunately the Rails Gods have dealt with this before. So like many other things in Rails, it’s easy to do. I want to store the colors in a simple Array.
Let’s modify the table.

  create_table "cars" do |t|
    t.column "make",  :string
    t.column "model", :string
    t.column "new_smell", :boolean
    t.column "colors", :text
  end

Notice I added the colors field with a type of :text. That’s because Rails will serialize my Ruby Array (or most any other object I specify) into YAML. And because it’s in the YAML format, it should be accessible from any other language that can read YAML. Ok, almost done. We just need to add a line to our model:

  class Car < ActiveRecord::Base
    serialize :colors, Array
  end
 

serialize is a class method that tells ActiveRecord to move the colors field back and forth between YAML and an Array. So in our example above, 1.) :colors tells it to use the colors field and 2.) Array (optional) tells it to check what I’m trying to store. So in our example if I try to pass a Hash to the colors field an exception is thrown.

Now I can use the Array on my model as normal:

 Car.create( :make => "Yugo", :model => "Hatchback",
                 :colors => ['red','yellow','magenta'] )
 c = Car.find_by_make("Yugo")

 puts "Available colors:"
 c.colors.each{ |color|
	puts "#{color} Yugo"
 }

And that’s it. Just two lines of code to add to your app! Check out the docs for more details.

Just a simple problem

It all started when I wanted to add the Rails default logger to some code I was working on. The code sits in the lib directory and I wanted to use the same logger the controllers and models use. Without thinking I threw a call to logger.info(”whatever”) in my code. Obviously that’s not going to work as I quickly got an error. Fortunately a quick google search turned up this. Basically to use the logger in my code I simply needed to call the constant RAILS_DEFAULT_LOGGER. Ok, great. Problem solved. Time to move on. But something caught my curiosity. How is Rails setting this constant and making it available to my Class ?

The plot thickens

So I decide to dig in. Luckily I start by looking at the code in initializer.rb. Rails::Initializer is called when your app boots up. It does most of the heavy lifting configuring the environment for Rails. Right around line 258, I find the answer to the constant mystery:

  Object.const_set "RAILS_DEFAULT_LOGGER", logger

Great. Ok now I can move on…but what the heck is

  Object.const_set

An aha moment

As it turns out the method const_set allows you to set a constant on the given object. In our case we’re setting the constant on Object - the parent of all classes in Ruby. So let’s see how this works…fire-up IRB and follow along:

Let’s start by seeing what constants Object already has. Ruby makes that easy:

  > Object.constants

As you can see (in your IRB terminal) Object comes with a large list of constants already. Ok let’s add our own

  > Object.const_set("WHOS_THE_MAN", "DAVE_IS")
  > Object.constants

Wow, you just added a constant to the supreme Ruby Object. Next, let’s see how we can get to this in our own classes:

  class Test
    def whos_the_man?
      puts WHOS_THE_MAN
    end
  end
  t = Test.new
  t.whos_the_man => DAVE_IS

So there you have it. A clever way to add constants that are available throughout your Ruby app. Of course you’re not limited to just adding to Object. Your Class can have it’s own constants that are scoped to it and its subclasses. You gotta love this language!