rails add index migration code example

Example 1: rails g migration add columns

$ rails generate migration AddDetailsToProducts part_number:string price:decimal

Example 2: how do I add index's to a migration in rails

rails generate migration AddUser_idColumnToTable user_id:string. 

class AddIndexToTable < ActiveRecord::Migration
  def change
    add_index :table, :user_id
  end
end