discourse/db/migrate/20120720013733_add_username_lower_to_users.rb

class AddUsernameLowerToUsers < ActiveRecord::Migration
  def up
    add_column :users, :username_lower, :string, limit: 20
    execute "update users set username_lower = lower(username)"
    add_index :users, [:username_lower], unique: true
    change_column :users, :username_lower, :string, limit: 20, null:false
  end
  def down
    remove_column :users, :username_lower
  end
end