A simple define_method example

Someone on #ruby-lang wanted to know how to define a method on a class when the method is first called. Here's a quick example.

require 'rubygems'
require 'spec'

module DynamicGetter
  def method_missing(name, *args)
    if(@attributes.has_key? name)
      self.class.class_eval do
       define_method(name){ @attributes[name] }
      end
      self.send(name, *args)
    else
      super
    end
  end
end

describe DynamicGetter do
  before(:each) do
  # Reference the class used in the specs
  # as an ordinary variable rather than a
  # named constant (@ooga instead of Ooga)
  # so that it can be created afresh for
  # each spec.
  # If we did class Ooga, the second spec
  # would fail because the first spec already
  # created the method #woot.

    @ooga_klass = Class.new(Object)
    @ooga_klass.class_eval do
      include DynamicGetter

      def initialize(attributes = {})
        @attributes = attributes
      end
    end
  end

  it "should know how to add a method to a class on first call" do
    o = @ooga_klass.new(:woot => 5)
    o.should_not respond_to(:woot)
    o.woot.should == 5
    o.should respond_to(:woot)
  end

  it "should raise a method not found exception if the attribute isn't present" do
    lambda{ @ooga_klass.new.woot }.should raise_error(NoMethodError)
  end
end

Another example is the Rails find_by_* methods, which are defined the first time you call them.
If, for some reason, you want to apply the example I've given on a per-instance basis, look at this post.

Related posts:

• Ruby blocks gotchas
• Loading classes from strings in Ruby
• Ruby's new as a factory
• Ruby's method_added object lifcycle hook
• Consistent interfaces, contrived examples and define_method for instances