Question

As part of an optimization, we don’t store our entire User in the session, instead we wrap the User inside a facade which contains only the basic attributes, such as id, username and admin?. Anything else is lazy-loaded (using method_missing). When this object is written to the session, it unloads the user prior to marshalling.

Basically:

class UserProxy
  attr_reader :id
  attr_reader :username

  def initialize(user)
    @user = user
    reload_attributes!
  end

  def admin?
    @admin
  end

  def self.model_name
    User.model_name
  end

  def method_missing(meth, *args, &block)
    @user ||= User.get!(id)
    @user.send(meth, *args, &block)
  ensure
    reload_attributes! # seems as good a place as any to keep the data fresh
  end

  def unset_user!
    @user = nil
    self
  end

  def ==(*args, &block)
    method_missing('==', *args, &block)
  end

  def !=(*args, &block)
    method_missing('!=', *args, &block)
  end

  def self._load(data) # custom unmarshalling logic
    UserProxy.new(ActiveSupport::JSON.decode(data)).unset_user!
  end

  def _dump(depth) # custom marshalling logic
    reload_attributes!
    ActiveSupport::JSON.encode({"id" => id, "username" => username, "admin" => admin})
  end

  private

    def reload_attributes!
      return if @user.nil?

      @id = @user["id"]
      @username = @user["username"]
      @admin = @user["admin"]
    end
end

This works great everywhere (it behaves just like the DataMapper user it wraps and we don’t have to hit the database to get the user on most page requests, since the important bits (key and username) are already loaded.

What it doesn’t play nicely with, is the path helpers Rails creates.

<%= url_for(@user) # should be /users/1234 %>

Initially it would error badly because it couldn’t figure out what the model_name was, so I fixed that by adding the class method model_name to my UserProxy class and just returning User.model_name and it now finds the correct routes, but in place of the user ID, it puts:

<a href="/users/%23%3CUserProxy:0xa4449cc%3E">Chris Corbyn</a>

I’m not sure where to look to fix this one. I’ve put debug calls in method_missing to see what is being invoked, but it’s not even receiving a call to to_param, like I’d expect it to.

Answer 1

I can’t believe I didn’t try this. I just added to_param to the proxy and delegated the call to the user object and it works. I didn’t realize all objects in Rails had a to_param method, model or not. I should probably do the same for to_key then!