indexes has_many :through

suppose have twin models, user city, assimilated third denote citypermission:

class citypermission < activerecord::base
  belongs_to :city
  belongs_to :user
end

class city < activerecord::base
  has_many :city_permissions
  has_many :users, :through => :city_permissions
end

class user < activerecord::base
  has_many :city_permissions
  has_many :cities, :through => :city_permissions
end

currently, i emanate join table, index table, controlling following emigration formula snippet:

create_table :city_permissions |t|
      t.integer :user_id, :city_id
      t.other_fields ...
end

add_index(:city_permissions, :user_id)
add_index(:city_permissions, :city_id)

are optimal indexes create? indexes grant discerning opening behind leading by join table, good discerning lookups within list itself, there improved way? reiterate bit differently, indexes, given city user instance variables category city user, grant city.users, city.city_permissions, user.cities, user.city_permissions perform equally well?