Virtus::MultiparameterAttributes (extracted as gem: https://github.com/sj26/virtus-multiparams)

virtus-multiparameter_attributes.rb

# Rails datetime_select and similar use multiparameter attributes which are
# these dawful things from the bowels of activerecord and actionpack. This
# module extends virtus models to coerce multiparameter attributes back together
# before assigning attributes.
#
# Here's the implementation for ActiveRecord:
#
#   https://github.com/rails/rails/blob/11fd052aa815ae0255ea5b2463e88138fb3fec61/activerecord/lib/active_record/attribute_assignment.rb#L113-L218
#
# and DataMapper:
#
#   https://github.com/datamapper/dm-rails/blob/5f3fb01a9d5f5adb17665a3bdabffd396d722e61/lib/dm-rails/multiparameter_attributes.rb
#
module Virtus::MultiparameterAttributes
  extend ActiveSupport::Concern

  included do
    attribute_set.singleton_class.send(:include, AttributeSetExtension)
  end

  module AttributeSetExtension
    extend ActiveSupport::Concern

    included do
      alias_method_chain :coerce, :multiparameter
    end

    def coerce_with_multiparameter(attributes)
      coerce_without_multiparameter(attributes).tap do |attributes|
        multiparams = Hash.new { |hash, key| hash[key] = {} }

        attributes.each do |(key, value)|
          if /\A(?<name>.+)\((?<offset>[0-9]+)(?<cast>[if])?\)\Z/ =~ key
            attributes.delete(key)

            unless attributes.include? name
              offset = offset.to_i
              value = value.send("to_#{cast}") if cast
              multiparams[name][offset] = value
            end
          end
        end

        multiparams.each do |name, values|
          array = Array.new(values.keys.max)

          values.each do |offset, value|
            array[offset - 1] = value
          end

          # Virus::Attribute has a primitive type which might be Date, Time, DateTime, etc.
          attribute = self[name]

          attributes[name] = if attribute.primitive <= Date || attribute.primitive <= Time
              # Basic convesion is enough, Virtus invokes `to_date[time]`
              # Also, lololol timezones
              Time.new(*array[0...6]) unless array[0...3].any?(&:blank?)
            else
              array
            end
        end
      end
    end
  end
end

Thanks for sharing this! How do you use it, after including the module in the virtus-powered class? Thanks!

I've release this as a gem with usage example:

https://github.com/sj26/virtus-multiparams