I know RSpec tests should typically be implementation/data agnostic, but I ran into a situation in a unit test where I needed to make sure that the results of a query returned only one of the two records that were created during the setup but without knowing which record that would be ahead of time. This was due to the fact that the scoped finder returned a random record. So, I wrote a little matcher that lets me assert that a given array contains only one of a set of specified elements.

# 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
view raw include_only_one_of.rb This Gist brought to you by GitHub.

You can use it like so:

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
view raw example_spec.rb This Gist brought to you by GitHub.