Has_many to has_many ... do I need to add add_index in both directions?

I am new to Rails. I am wondering if I need add_index for both migrations?

I am trying to identify users and events. Each user can have many events, and each event can have many users. so I would do something like this:

class User < ActiveRecord::Base
  attr_accessor :password
  attr_accessible :name, :email, :password, :password_confirmation

  has_many :events
.
.
.

and then this:

class Event < ActiveRecord::Base
  attr_accessible :name

  has_many :users
.
.
.

I need to add add_index for both:

add_index :events, :user_id

and then

add_index :users, :event_id

Are these assumptions correct?