Can't cache model using Dalli

Question

I'm having a hard time cacheing an object.

tracked_point.rb

class TrackedPoint < ActiveRecord::Base
  attr_accessible :recorded_at, :worker_id, :worker, :x, :y, :z, :geom, :location_id, :location, :frequency
  attr_accessor :x, :y
  belongs_to :worker
  belongs_to :location, counter_cache: true

  has_many :infraction_locations
  has_many :infractions, through: :infraction_locations

  validates :location_id, presence: true

  set_rgeo_factory_for_column(:geom, FACTORY)

  after_save :check_infractions

  def check_infractions
    self.write_to_cache
    self.zones.each do |zone|
      zone.write_to_cache
      ZoneWorker.perform_async(zone.id, self.id)
    end
    PrivatePub.publish_to "/tracked_points/new", :tracked_point => {x: self.geom.x, y: self.geom.y, z: self.geom.z, worker_id: self.worker_id}
    Rails.cache.write([self.cached_worker, "cached_previous_tracked_point_id"], self.id)
  end

  def write_to_cache
    Rails.cache.write([self.class.name, id], self)
  end
end

When a tracked_point is created, my check_infractions callback happens (technically after_save, but that's because I want it to run on update as well).

console:

1.9.3-p392 :001 > worker = Worker.find 88
1.9.3-p392 :002 > worker.tracked_points.create(x:3, y:5, location_id: 15, recorded_at: Time.now)

This results in an error:

You are trying to cache a Ruby object which cannot be serialized to memcached.

This happened at the line where self.write_to_cache was called. So I manually tried to run that code:

1.9.3-p392 :007 > tp = worker.tracked_points.create(x:3, y: 3, location_id: 15, recorded_at: Time.now)
1.9.3-p392 :007 > tp.write_to_cache

Same error. Let's try just creating a TrackedPoint.

1.9.3-p392 :008 > tp2= TrackedPoint.create(worker_id: 88, x: 5, y: 5, recorded_at: Time.now, location_id: 15)

Same error.

If I pull the last TrackedPoint from the database, however, I can cache it:

1.9.3-p392 :012 > tp = TrackedPoint.last
1.9.3-p392 :013 > tp.write_to_cache
  true

Any ideas why cacheing in my after_save callback and object creation is failing but if I pull the object from the database I can cache it?

• ruby 1.9.3-p392
• rails 3.2.13
• dalli 2.6.4