RSpec matcher for delegations

delegate_matcher.rb

# RSpec matcher to spec delegations.
#
# Usage:
#
#     describe Post do
#       it { should delegate(:title).to(:name) } # post.title => post.name
#       it { should delegate(:month).to(:created_at) } # post.month => post.created_at
#       it { should delegate(:author_name).to(:author, :name) } # post.author_name => post.author.name
#     end

RSpec::Matchers.define :delegate do |method|
  match do |delegator|
    @method = method
    @delegator = delegator

    @delegator.stub_chain(@to) { :return_value }

    @delegator.send(@method) == :return_value
  end

  description do
    "delegate :#{@method} to #{pretty_to}"
  end

  failure_message_for_should do |text|
    "expected #{@delegator} to delegate :#{@method} to #{pretty_to}"
  end

  failure_message_for_should_not do |text|
    "expected #{@delegator} not to delegate :#{@method} to #{pretty_to}"
  end

  def pretty_to
    @to.join('.')
  end

  chain(:to) { |*to| @to = to }
end