You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 20210907154701_create_activity_costs.rb
    path of file: ./repos/ekylibre/db/migrate
    
    the code of the file until where you have to start completion: class CreateActivityCosts < ActiveRecord::Migration[5.0]
  def change
    create_table :activity_cost_outputs do |t|
        t.references :activity, null: false, index: true, foreign_key: true
        t.references :variant, null: false, index: true
        t.references :variant_unit, null: fa
    
  def change
    create_table :activity_cost_outputs do |t|
        t.references :activity, null: false, index: true, foreign_key: true
        t.references :variant, null: false, index: true
        t.references :variant_unit, null: false, index: true
        t.string :name
        t.stamps
      end
      add_foreign_key :activity_cost_outputs, :product_nature_variants, column: :variant_id
      add_foreign_key :activity_cost_outputs, :units, column: :variant_unit_id
  end