Q:

Error Adding AUTO_INCREMENT Column to Rails Model - MySQL

I currently have an existing model, Document, that needs a new, auto-incrementing column. Unfortunately, I am getting errors during the migrate. 
The steps I have taken are:
ruby script/generate migration add_index_column_to_Document
which properly generated the empty .rb titled 
20121220182429_add_index_column_to_document.rb
Next, I edited the file to look like:
class AddIndexColumnToDocument < ActiveRecord::Migration
  def self.up
    execute 'ALTER TABLE documents ADD index INT NOT NULL AUTO_INCREMENT PRIMARY KEY'
  end

  def self.down
    execute 'ALTER TABLE documents DROP index'
  end
end

Then I executed the migration with rake db:migrate and got the following error:
==  AddIndexColumnToDocument: migrating =======================================
-- execute("ALTER TABLE documents ADD index INT NOT NULL AUTO_INCREMENT PRIMARY KEY")
rake aborted!
An error has occurred, all later migrations canceled:

Mysql::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'INT NOT NULL AUTO_INCREMENT PRIMARY KEY' at line 1: ALTER TABLE documents ADD index INT NOT NULL AUTO_INCREMENT PRIMARY KEY

The MySQL version, according to apt-cache show mysql-server is: 5.5.28-0ubuntu0.12.04.2
Unfortunately for me, my SQL code matches that of all examples I can find online, so I am unsure as to why it is not working. Thanks in advance for any help you may provide.

A:

Try escaping the word 'index' which is reserved.  Even better, let Rails do it for you:
class AddIndexColumnToDocument < ActiveRecord::Migration
  def change
    add_column :documents, :index, :primary_key
  end
end

