A simple form object built on top of virtus.

example.rb

class Filter
  include FormObject

  # Proc because Date.yesterday changes every day :)
  attribute :from, Date, default: Proc.new { Date.yesterday }
  attribute :to, Date, default: Proc.new { 1.month.from_now - 1.day }
end

# in controller
@filter = Filter.new(params[:filter])
@orders = Orders.between(@filter.from, @filter.to)

# in view
@filter.from # always a date, either a user entered date or the default date

form_object.rb

# Ensures values are either the type you expect, a default value or nil.
#
# Virtus is built to handle data from a database where it's either well formed or nil. We need
# to tweak it's return values a little bit for it to handle form data. See https://github.com/solnic/virtus/issues/99.
module FormObject
  include Virtus

  module ClassMethods
    def attribute(name, type, opts = {})
      super(name, type, opts)
      define_method(name) do
        data = super()
        if data.is_a?(type)
          data
        elsif opts[:default]
          opts[:default].call
        else
          nil
        end
      end
    end
  end

  def self.included(descendant)
    super
    descendant.extend FormObject::ClassMethods
  end
end

form_object_spec.rb

require 'spec_helper'
require 'form_object'

class Filter
  include FormObject
  attribute :from, Date, default: Proc.new { Date.yesterday }
  attribute :number, Integer
end

describe FormObject do
  it "can handle data from a form" do
    filter = Filter.new(from: "2012-01-15")
    filter.from.should be_kind_of(Date)
    filter.from.should == Date.parse("2012-01-15")
  end

  it "defaults" do
    filter = Filter.new
    filter.from.should == Date.yesterday
  end

  it "defaults when it can't parse a value" do
    filter = Filter.new(from: "2012-01-55")
    filter.from.should == Date.yesterday
  end

  it "returns nil when it can't parse the value and there is no default" do
    filter = Filter.new(number: "2012-01-55")
    filter.number.should be_nil
  end

  it "returns nil when there is no default" do
    filter = Filter.new
    filter.number.should be_nil
  end
end

This fails if the type is an Array of something, like attribute :addresses, Array[Address]