polymorphic_urlの中身で使われている build_named_route_callのソースを貼付けておきます。

ruby>>
def build_named_route_call(records, namespace, inflection, options = {})
unless records.is_a?(Array)
record = extract_record(records)
route = ''
else
record = records.pop
route = records.inject("") do |string, parent|
if parent.is_a?(Symbol) || parent.is_a?(String)
string << "#{parent}"
else
string << "#{RecordIdentifier.send("plural_class_name", parent
)}".singularize
string << ""
end
end
end

    if record.is_a?(Symbol) || record.is_a?(String)
      route << "#{record}_"
    else
      route << "#{RecordIdentifier.__send__("plural_class_name", record)}"
      route = route.singularize if inflection == :singular
      route << "_"
    end

    action_prefix(options) + namespace + route + routing_type(options).to_s
  end

<<--