An RSpec2 matcher that asserts that only one of the passed elements is present in the source array

example_spec.rb

require 'include_only_one_of'

describe "SomeTest" do
  it "should return only one wide format promotion if available" do
    first_wide_promotion  = Factory(:promotion, :brand => @brand, :format => Promotion::FORMATS[:wide])
    second_wide_promotion = Factory(:promotion, :brand => @brand, :format => Promotion::FORMATS[:wide])
    @brand.featured_promotions.should include_only_one_of(first_wide_promotion, second_wide_promotion)
  end
end

include_only_one_of.rb

# Inspired by http://blog.nicksieger.com/articles/2011/01/20/rspec-2-matcher-fun
RSpec::Matchers.define :include_only_one_of do |*elements|
  match do |container|
    @included = []
    elements.flatten.each do |e|
      @included << e if container.include?(e)
    end
    @included.count == 1
  end

  failure_message_for_should do |container|
    "expected array of #{container.length} elements to include only one member.\n" +
      "Found #{@included.flatten.inspect}"
  end
end