neat_evil_and_doomed.rb

class Neat < ActiveRecord::Base
  class << self
    def new(params = {})
      if self == Search
        if params.with_indifferent_access.include?(:evil_quota)
          Evil.new(params)
        elsif params.include?(:doom_level)
          Doomed.new(params)
        else
          super
        end
      else
        super
      end
    end
  end
end

class Evil < Neat
end

class Doomed < Neat
end

n = Neat.new(:evil_quota => 3.75)
n.is_a?(Neat) # => true
n.is_a?(Evil) # => true