class DeepPreloader::PolymorphicSpec

def merge!(other)
  case other
  when nil
    return
  when DeepPreloader::PolymorphicSpec
    other.specs_by_type.each do |k, v|
      if specs_by_type[k]
        specs_by_type[k].merge!(v)
      else
        specs_by_type[k] = v
      end
    end
  else
    raise ArgumentError.new("Cannot merge #{other.class.name} into #{self.inspect}")
  end
  self
end