I would like to use transactions when importing csv files in rails.(If an error occurs, cancel all line changes and return to the original screen to display the error.)

$rails-v 
5.0.0

def import
  if params[:csv_file].blank?
    redirect_to action: 'index', error: 'Please select CSV to load'
  elsif File.extname(params[:csv_file].original_filename)!=".csv"
    redirect_to action: 'index', notice: 'Only csv files can be loaded'
  else
    num = Cost.import_by_csv (params[:csv_file])
    redirect_to action: 'index', notice: "# {num.to_s} data information added/updated"
  end
end

require'csv'
class Cost<ActiveRecord::Base

  # Register the contents of the csv file in DB
  def self.import_by_csv(file)
    imported_num = 0
    open(file.path, 'r:cp932:utf-8', undef::replace) do | f |
      csv=CSV.new(f, :headers=>:first_row)
      caches = Cost.all.index_by (&:code)
      csv.each do | row |
        next if row.header_row?

  # Convert CSV Row Information to HASH
        table = Hash [[row.headers, row.fields].transpose]

  # Registered Data Information
  # Create if not registered
        cost=caches [table['code']]

        if cost.nil?
           cost=new
        end
  # data information update
        cost.attributes=table.slice(*table.except(:created_at,:updated_at).keys)

  # Save for validation ok
        if cost.valid?
          cost.save!
          imported_num+=1
        end
      end
    end

  # return the number of updates
    imported_num
  end
end

=form_tag import_costs_path, method::post, multipart:true do
  = file_field_tag —csv_file
  = submit_tag 'CSV Read'

Rails.application.routes.drawdo
  resources —costs do
  post:import, on::collection
end

Cost.transaction do 
  ------ Code -----
end