have_a_matcher.rb

module HaveAMatcher
  class HaveA
    def method_missing(attribute)
      @attribute ||= attribute
      self
    end
    def matches?(target)
      @target = target
      raise ArgumentError, "No attribute specified for have(:a)" unless @attribute
      @value = @target.__send__(@attribute)
      !(@value.nil? || @value.empty?)
    end
    def failure_message
      "expected #{@target} to have #{an} #{@attribute}, but its #{@attribute} was #{@value.inspect}."
    end
    def negative_failure_message
      "expected #{@target} not to have #{an} #{@attribute}, but its #{@attribute} was #{@value.inspect}."
    end
    def an
      vowels  = [?a, ?e, ?i, ?o, ?u]
      vowels += [?A, ?E, ?I, ?O, ?U]
      if vowels.include? @attribute.to_s[0]
        "an"
      else
        "a"
      end
    end
  end

  def have(n=nil)
    if n.nil?
      HaveA.new
    else
      super
    end
  end
  alias :have_a  :have
  alias :have_an :have
end