First and only

first_and_only.rb

module Enumerable
  def first_and_only!
    fail(FirstAndOnly::LengthNotOne, count) if first(2).count != 1
    first
  end

  module FirstAndOnly
    LengthNotOne = Class.new(StandardError)
  end
end

usage.markdown

FirstAndOnly

One Thing

require first_and_only

[:thing].first_and_only! # => :thing

Too Many Things

require first_and_only

[1,2].first_and_only!
# => Enumerable::FirstAndOnly::LengthNotOne: 2

Not Enough Things

require first_and_only

[].first_and_only!
# => Enumerable::FirstAndOnly::LengthNotOne: 0

Works With ActiveRecord

require first_and_only

MyModel.count  # => 0

MyModel.where(some: "query").first_and_only!
# => Enumerable::FirstAndOnly::LengthNotOne: 0

All the ways I tried to include a module into a module...

The eigen class strikes again: https://www.ruby-forum.com/topic/109820. That explains why I couldn't get it to work.