A recent thread on the Rails mailing list grappled my attention: how do association accessor methods work, how do attribute accessor methods work, and what’s the difference (in ActiveRecord)?

Association accessor methods are things like story.author when Story belongs_to :author. An attribute accessor method would be something like story.title when the stories database table has a title column. You’re familiar, I assume.

The question arose from a failed attempt to override an association accessor method. Something like this did not work:

class Story < ActiveRecord::Base
  belongs_to :author

  def author
    auth = super
    auth.name
  end
end

Obviously I want stories.author to return the name of a particular story’s author rather than the associated Author object. Remember that the super method calls the current method on the class parent. I’m trying to call the author method, then, on ActiveRecord::Base (hoping it will return the associated author object like normal). Rails, unfortunately, is having none of it:

>> story = Story.find(:first)
=> #<Story:0x279f9e0 ...>
>> story.author
NoMethodError: super: no superclass method `author'

Alright, I should have seen that coming. There’s this method_missing magic all over ActiveRecord… maybe author is a magical method? If that’s the case a question becomes apparent: if my parent class doesn’t have the method I’m calling super from, will its method_missing be hit? Let’s see.

$ cat animals.rb
class Animal
  def method_missing(method, *args)
    "Sorry, I don't have #{method}."
  end
end

class Dog < Animal
  def bark
    super
  end
end

tanner = Dog.new
puts tanner.bark

I expect to get a super: no superclass method `bark’ error. After all, isn’t this basically the same situation I’m running into with my authors method?

$ ruby animals.rb
Sorry, I don't have bark.

Interesting. So super did work as expected and hit method_missing. What’s really going on here?

The truth’s that authors is not really a method_missing trick. It’s a real method defined on my Story class. Where does it get defined? When belongs_to is called. Like much of Rails’ sugar, belongs_to is a class method of ActiveRecord::Base. When you call it in a subclass, stuff happens.

The adventurous can peek into active_record/associations.rb around line 646 to see how belongs_to is defined. Here’s the cliffs: when belongs_to is called, ActiveRecord defines reader and writer methods on the calling class. Now it’s starting to make sense: author is added to my Story class when belongs_to is called. It’s not a method_missing trick and doesn’t exist on ActiveRecord::Base. Knowing this, how do we get our original example to work?

Enter: alias. You may have seen it lurking around, all meta-like and somewhat shady. alias, more or less, copies a method. That’s it. It’s useful when you want to override a method but still keep the original method around. The technique is sometimes called “method chaining.” Say it: “meth-odd-chain-ing.”

Solution:

class Story < ActiveRecord::Base
  belongs_to :author

  alias :real_author :author
  def author
    auth = real_author
    auth.name
  end
end

Now?

>> story = Story.find(:first)
=> #<Story:0x279f9e0 ...>
>> story.author
=> "Christoe Jefferson"

That feels great. But I still have a question: what about, say, story.title? Does ActiveRecord define all of my attribute accessor methods in the same way it defines my association accessor methods?

All told, attribute accessor methods are indeed method_missing magic. Get pumped: super will work when overriding them.

class Story < ActiveRecord::Base
  belongs_to :author

  def title
    "This story's title is: " + super
  end
end

So close, so close…

>> story = Story.find(:first)
=> #<Story:0x279f9e0 ...>
>> story.title
=> "This story's title is: Accessor Missing"

Paydirt.

That overachiever Josh Susser has a great write up on how Rails uses method_missing to allow dynamic finders, like Story.find_by_title(‘Accessor Missing’). Check it out if you haven’t already. There’s also that dude who can’t seem to keep those eels out of his hovercraft—he’s got a great post on using alias.