What's the correct syntax for remove_index in a Rails 3.1.0 migration?

Question

I\'m in the process of adding Devise to an existing Rails app, with a Users table already defined. The devise generator pushed out the following migration:

class         


        

Answer 1

I'd like to expand on @iWasRobbed's answer. If you have index on just single column then worrying about remove_index doesn't make sense since (just an assumtion!) the DB should be smart enough to cleanup the resources used by that index. But in case you have multiple columns index removing the column will reduce index to still existing columns, which is totally sensible thing to do, but kind of shows where you might want to use remove_index explicitely.

Just for illustration - migration below has that flaw that after being applied up and down it will leave the unique index on email (meaning the down part is not doing its job properly)

class AddIndexes < ActiveRecord::Migration
  def up
    add_column :users, :action_name, :string
    add_index  :users, [:email, :action_name], unique: true
  end

  def down
    remove_column :users, :action_name
  end
end

Changing the down block to

  def down
    remove_index :users, [:email, :action_name]
    remove_column :users, :action_name
  end

will fix that flaw and allow the migration to correctly return DB to the previous state with rake db:rollback