Improved Rails’ delegate method

Filed in: Rails, Ruby Add comments

The Rails framework includes a simple “delegate” method, which is mixed in the models, and can be used in order to delegate methods to other classes. However it’s limited at least in the following:
1. It doesn’t prevent “train wrecks” – e.g. if the class that the method is delegated to is nil, then there will be an attempt to call this method on the nil class.
2. It doesn’t allow renaming of the methods, e.g. if you have a method called “name” in your class, and a method called “name” in other class you want to delegate to, you have no means of doing this.
The implementation below remedies these shortcomings.

class Module
  def delegate(*methods)
    options = methods.pop
    unless options.is_a?(Hash) && to = options[:to]
      raise ArgumentError, "Delegation needs a target. Supply an options hash with a :to key as the last argument (e.g. delegate :hello, :to => :greeter)."
    end
 
    prefix = options[:prefix] || ''
    orig_prefix = prefix
    if prefix != true and !prefix.empty? 
      prefix += '_'
    end
 
    methods.each do |method|
      prefix = to.to_s + '_' if orig_prefix == true
      module_eval(<<-EOS, "(__DELEGATION__)", 1)
        def #{prefix}#{method}(*args, &block)
          return nil unless #{to}
          #{to}.__send__(#{method.inspect}, *args, &block)
        end
      EOS
    end
  end
end

You can use it like this:

delegate :name, :to=> :other, :prefix=>true

and it will delegate a method “name” in “other” class as “other_name” in your class.
Or, alternatively

delegate :name, :to=> :other, :prefix=>'some'

will delegate “some_name” to class “other”, method “name”.

2 Responses to “Improved Rails’ delegate method”

  1. jolly jim Says:
    January 19th, 2009 at 05:43

    Awesome! This works with Merb/Datamapper as well with no changes required.

    Reply

  2. Emerson Macedo Says:
    November 27th, 2009 at 05:16

    A DRY way to apply Law of Demeter with demeter gem

    http://github.com/emerleite/demeter
    http://gemcutter.org/gems/demeter

    Reply

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