= form_tag upload_url, :multipart => true do
  %label{:for => "file"} File to Upload
  = file_field_tag "file"
  = submit_tag

class CustomersController < ApplicationController  
  [...] 
  def import
    if request.post? && params[:file].present?
      infile = params[:file].read
      n, errs = 0, []
 
      CSV.parse(infile) do |row|
        n += 1
        # SKIP: header i.e. first row OR blank row
        next if n == 1 or row.join.blank?
        # build_from_csv method will map customer attributes & 
        # build new customer record
        customer = Customer.build_from_csv(row)
        # Save upon valid 
        # otherwise collect error records to export
        if customer.valid?
          customer.save
        else
          errs << row
        end
      end
      # Export Error file for later upload upon correction
      if errs.any?
        errFile ="errors_#{Date.today.strftime('%d%b%y')}.csv"
        errs.insert(0, Customer.csv_header)
        errCSV = CSV.generate do |csv|
          errs.each {|row| csv << row}
        end
        send_data errCSV,
          :type => 'text/csv; charset=iso-8859-1; header=present',
          :disposition => "attachment; filename=#{errFile}.csv"
      else
        flash[:notice] = I18n.t('customer.import.success')
        redirect_to import_url #GET
      end
    end
  end
  [...]
end

class Customer < ActiveRecord::Base
  scope :active, where(:active => true)
  scope :latest, order('created_at desc')
 
  def self.csv_header
    "First Name,Last Name,Email,Phone,Mobile, Address, FAX, City".split(',')
  end
 
  def self.build_from_csv(row)
    # find existing customer from email or create new
    cust = find_or_initialize_by_email(row[2])
    cust.attributes ={:first_name => row[0],
      :last_name => row[1],
      :email => row[3],
      :phone => row[4],
      :mobile => row[5],
      :address => row[6],
      :fax => row[7],
      :city => row[8]}
    return cust
  end
 
  def to_csv
    [first_name, last_name, email, phone, mobile, address, fax, city]
  end
end