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: 20121127122903_create_impressions.rb
    path of file: ./repos/qaror/db/migrate
    
    the code of the file until where you have to start completion: class CreateImpressions < ActiveRecord::Migration
 
    
  def self.up
    create_table :impressions do |t|
      t.references :impressionable, :polymorphic => true
      t.references :user
      t.string :ip
      t.datetime :created_at
    end

    add_index :impressions, :ip
    add_index :impressions, :user_id
    add_index :impressions, [ :impressionable_type, :impressionable_id]
  end