Cacheing has_many through:

fail.console

p = Person.create
p.items << Item.create
Rails.cache.write(Time.now, p)
=> false
p.reload
Rails.cache.write(Time.now, p)
=> <truthy>

item.rb

class Item < ActiveRecord::Base
  attr_accessible :description

  has_many :person_items
  has_many :people, through: :person_items
end

person.rb

class Person < ActiveRecord::Base
  attr_accessible :name

  has_many :person_locations
  has_many :locations, through: :person_locations

  has_many :person_items
  has_many :items, through: :person_items
end

person_item.rb

class PersonItem < ActiveRecord::Base
  attr_accessible :item_id, :person_id

  belongs_to :item
  belongs_to :person
end