_form.html.haml

= f.inputs  :name => :code_promo, :class => 'inputs foldable' do
  = f.input :validity_start, :as => :datetime, :include_blank => false
  = f.input :validity_end,:as => :datetime, :include_blank => false

deductions_controller.rb

class DeductionController < BaseController
  before_filter :current_deduction
  helper_method :current_deduction

  def create
      raise current_deduction.inspect
      if current_deduction.save
        redirect_to admin_deductions_url
      else
        flash[:error] = "Veuillez corriger les erreurs "
        render :new
      end
  end
      def current_deduction
      args = params[:deduction].present? ?  params[:deduction]  : {:validity_start => DateTime.now, :validity_end => DateTime.now + 1.year}
      @current_deduction=  params[:id].present?  ? Deduction.not_hidden.includes(:deduction_codes).find(params[:id]) : Deduction.new(args)
    end
end

mon raise

#<Deduction id: nil, validity_end: "2012-03-17 00:00:00", validity_start: "2012-03-12 00:00:00">

Parameters:

{"utf8"=>"✓",
 "authenticity_token"=>"+YCcpyg=",
 "deduction"=>{
 "validity_start(3i)"=>"12",
 "validity_start(2i)"=>"3",
 "validity_start(1i)"=>"2012",
 "validity_start(4i)"=>"10",
 "validity_start(5i)"=>"12",
 "validity_end(3i)"=>"17",
 "validity_end(2i)"=>"6",
 "validity_end(1i)"=>"2013",
 "validity_end(4i)"=>"14",
 "validity_end(5i)"=>"12"},
 "commit"=>"Create"}

schema.rb

  create_table "deductions", :force => true do |t|

    t.datetime "created_at",                                  :null => false
    t.datetime "updated_at",                                  :null => false

  end