Rspec delegation matcher

delegates_matcher.rb

# RSpec matcher to spec delegations.
#
# Usage:
#
#     describe Post do
#       # post.name -> post.author.name
#       it { should delegate(:name).to(:author) }
#
#       # post.author_name -> post.author.name
#       it { should delegate(:author_name).to(:author).as(:name) }
#     end

RSpec::Matchers.define :delegate do |method|
  match do |delegator|
    @method = method
    @delegator = delegator
    as = @as || @method
    begin
      @delegator.send(@to)
    rescue NoMethodError
      raise "#{@delegator} does not respond to #{@to}!" 
    end
    @response = double('response')
    @receiver = double('receiver')
    @receiver.stub(as => @response)
    @delegator.stub(@to => @receiver)
    @delegator.send(@method) == @response
  end

  description do
    "delegate :#{@method} to its #{@to}#{@as ? " as #{@as}" : ''}"
  end

  failure_message_for_should do |text|
    "expected #{@delegator} to delegate :#{@method} to its #{@to}#{@as ? " as #{@as}" : ''}"
  end

  failure_message_for_should_not do |text|
    "expected #{@delegator} not to delegate :#{@method} to its #{@to}#{@as ? " as #{@as}" : ''}"
  end

  chain(:to) { |receiver| @to = receiver }
  chain(:as) { |as| @as = as }

end