Marshalling MongoMapper Documents

Conventional wisdom has it you shouldn't store models in your Rails sessions. Conventional wisdom is right: Marshalling complex objects can be slow; Session objects can become stale; Sessions can balloon up too large for cookie storage, limiting your scaling options.

However, the rule that models don't belong in the session is trumped by the rules that POSTs should yield GETs and GETs should not mutate state. Every page should be reloadable, without an annoying pop-up asking if you want to resubmit a form. POSTs are allowed to be dangerous. POSTs are allow to charge your credit card, delete records or log you out. So, when creating or editing a model, if the model fails to validate, I refuse to display an error page as a direct result of a POST. Instead, the user's browser is redirected back to the new or edit page they were on and the form is redisplayed with helpful error messages. Indeed, this has become a standard Rails pattern.

Which brings us to my most-recent roadblock. MongoDB is my new favorite toy and MongoMapper is a promising stand-in for ActiveRecord. However, when I employed this standard Rails pattern:

def new
 @user = flash[:user] || User.new
end

def create
 @user = User.new params[:user]
 if @user.save
   redirect_to user_path(@user)
 else
   flash[:user] = @user
   redirect_to new_user_path
 end
end

I was stymied by this error:

TypeError: singleton can't be dumped

Some chain of references from the model evidently connects it to a singleton object. MongoMapper is still under heavy development, so I'm confident at some point this will be resolved, but meanwhile the following monkey patch appears to do the trick:

module MongoMapper
 module Document
   def marshal_dump
     instance_variable_names.inject({}) { |m, name| m[name] = instance_variable_get(name); m }
   end

   def marshal_load(values)
     values.each_pair { |k, v| instance_variable_set(k, v) }
   end
 end
end

I stuck this code in a file called mongo_marshalling.rb in config/initializers.