Ruby: Add a uniqueness key to a db.

Ruby: Uniqueness Key

# rails generate migration add_email_uniqueness_index

class AddEmailUniquenessIndex < ActiveRecord::Migration
  def self.up
    # add_index to :users. to :email, add a unique constriction.
    add_index :users, :email, :unique => true
  end

  def self.down
    remove_index :users, :email
  end
end