I often need this in specs, sometimes I'd like to include this in code as well. The idea is to ensure that the query returns really only single result

rails_uniq_finder_extension.rb

module FinderExtensions
  def find_uniq!(cond)
    res = where(cond)
    res.first.tap do |r|
      raise ActiveRecord::RecordNotFound unless r
      raise ActiveRecord::ActiveRecordError, "multiple records found" if res[1]
    end
  end
end

ActiveRecord::Base.send(:extend, FinderExtensions)