Today's noun is: Reification

Posted by Piers Cawley on Aug 8, 2007

Reification: The mental conversion of a person or abstract concept into a thing. Also, depersonalization, esp. such as Marx thought was due to capitalist industrialization in which the worker is considered as the quantifiable labour factor in production or as a commodity. - OED

In the sense that the OED has it, I’m not what you could call a fan of reification. At work, we have a rule that anybody who starts talking about ‘resources’ when they mean ‘people’ gets a (verbal) slap.

However, in OO circles (or maybe just in my head), reification is a good thing. It’s the process of taking something abstract and turning it into a ‘real’ object. Usually, the word gets used for big things like turning an intractable method into an object as a step on the way to refactoring that method. I tend to use it in a slightly broader sense. For me, reification is the process of turning something (a method or a data structure usually) into a full blown object with its own behaviour.

Back when I was working on Pixie (a cunning, but weird, object persistence tool written in Perl) we had a data structure which was used for keeping track of managed objects. It started life as a hash. Everything was fine at first, but over time we ended up with more and more code being repeated across the codebase that was concerned with manipulating the cache hash. So, we replaced the hash with a new object and pulled all the repeated code into methods on that object, which gave us cleaner code to extend, and a strong feeling that we should have turned the cache into an object much earlier in the game. (By leaving it so long, we had a lot more code to move about, some of it in fairly obscure places; tracking down the last bit took a while.)

Data structures like hashes and arrays are really useful in languages that have them. The catch is, they have this habit of acquiring code. When this starts to happen, it’s time to reify - to replace the hash with a task specific object. In Ruby, it’s easy enough to inherit from Hash, but Hash comes with a pile of methods that probably aren’t relevant to your particular need. Generally it’s better to delegate. The first cut doesn’t have to be that complicated, just decorate the hash with a new class and initialize an instance of the class at the point where you had just made the hash.

Once that’s done, you can go through your code and move the bits that treat the hash as a data structure onto your new class. As you gather all the common behaviour to the new class, you’ll start to see places where you can improve code quality by merging common behaviours, replacing complex conditionals with polymorphism (you’ll probably have to introduce a factory method if you do that) and pulling hash keys out into instance variables.

Stalled reification

Reifying your data structure isn’t an end in itself, it’s a step along the way as you refactor your code.

There’s an example of a stalled reification to be found in ActionController::Routing::Resources. Consider the implementations of map_resource and map_singleton_resource, which are the worker methods used whenever you do a map.resource or map.resources in your routes.rb.

def map_resource(entities, options = {}, &block)
  resource = Resource.new(entities, options)

  with_options :controller => resource.controller do |map|
    map_collection_actions(map, resource)
    map_default_collection_actions(map, resource)
    map_new_actions(map, resource)
    map_member_actions(map, resource)

    if block_given?
      with_options(:path_prefix => resource.nesting_path_prefix, &block)
    end
  end
end

def map_singleton_resource(entities, options = {}, &block)
  resource = SingletonResource.new(entities, options)

  with_options :controller => resource.controller do |map|
    map_collection_actions(map, resource)
    map_default_singleton_actions(map, resource)
    map_new_actions(map, resource)
    map_member_actions(map, resource)

    if block_given?
      with_options(:path_prefix => resource.nesting_path_prefix, &block)
    end
  end
end

There’s a lot of repetition there. The only differences are the classes of the resource object, name of the second function called in the with_options block. If we take a look at, map_collection_actions things start to look even fishier. Here’s map_collection_actions, for example:

def map_collection_actions(map, resource)
  resource.collection_methods.each do |method, actions|
    actions.each do |action|
      action_options = action_options_for(action, resource, method)
      map.named_route("#{resource.name_prefix}#{action}_#{resource.plural}", "#{resource.path};#{action}", action_options)
      map.named_route("formatted_#{resource.name_prefix}#{action}_#{resource.plural}", "#{resource.path}.:format;#{action}", action_options)
    end
  end
end

resource.collection_methods.each? Let’s see what happens if we the various map_foo_actions methods into methods on ActionController::Resources::Resource. While we’re about it, we can rename map_default_collection_actions to map_default_actions on Resource, and map_default_singleton_actions to map_default_actions on SingletonResource, which inherits from Resource. map_collection_actions becomes:

def map_collection_actions(map)
  collection_methods.each do |method, actions|
    actions.each do |action|
      map.with_options(action_options_for(action, method)) do |m|
        m.named_route("#{name_prefix}#{action}_#{plural}",
                      "#{path};#{action}")
        m.named_route("formatted_#{name_prefix}#{action}_#{plural},
                      "#{path}.:format;#{action}")
      end
    end
  end
end

(we move action_options_for onto resource as well, of course).

Once we’ve moved the various mapping helpers onto the resource classes, we can revisit map_resource and map_singleton_resource

def map_resource(entities, options={}, &block)
  resource = Resource.new(entities, options)

  with_options(:controller => resource_controller) do |map|
    resource.map_collection_actions(map)
    resource.map_default_actions(map)
    resource.map_new_actions(map)
    resource.map_member_actions(map)
  end

  if block_given?
    with_options(:path_prefix => resource.nesting_path_prefix, &block)
  end
end

def map_singleton_resource(entities, options={}, &block)
  resource = SingletonResource.new(entities, options)

  with_options(:controller => resource.controller) do |map|
    resource.map_collection_actions(map)
    resource.map_default_actions(map)
    resource.map_new_actions(map)
    resource.map_member_actions(map)
  end

  if block_given?
    with_options(:path_prefix => resource.nesting_path_prefix, &block)
  end
end

And now, we no longer have two method bodies that look very similar, apart from the resource class, we have to methods that look identical apart from the resource class. So, if we pull out the common bits and put them onto Resource, like so:

class ActionController::Resources::Resource
  def install_routes_in(map, &block)
    map.with_options(:controller => controller) do |m|
      map_collection_actions(m)
      map_default_actions(m)
      map_new_actions(m)
      map_member_actions(m)
    end

    if block_given?
      map.with_options(:path_prefix => nesting_path_prefix, &block)
    end
  end
end

Then map_resource and map_singleton_resource become

def map_resource(entities, options = {}, &block)
  Resource.new(entities, options).install_routes_in(self)
end

def map_singleton_resource(entities, options = {}, &block)
  SingletonResource.new(entities, options).install_routes_in(self)
end

Where’s the benefit?

Apart from making the active_record/lib/resources.rb a bit shorter (a laudable result in itself), where’s the benefit here?

From my own experience of implementing datestamped_resource, a routing plugin that we use in Typo, it makes the life of anyone writing a resource like routing helper for Rails a great deal easier. With datestamped_resource I ended up subclassing ActionController::Resources::Resource, doing the refactoring I’ve outlined here, but leaving the original Rails methods where they were and just implementing the ‘moved’ methods on DatestampedResource (well, not quite, map_collection_actions is pretty different from the default Resource implementation, but the other actions are pretty much the same.

In another project I’m working on, I’m trying to retain meaningful urls with (potentially) deep resource nesting, and it’d be really handy to have an inflected_resource route helper. The problem with using a meaningful to_param on your models is, avoiding permalinks that share a name with your actions. You could set up validations so that, say, ‘new’ is an illegal permalinks, but it’s clumsy.

However, if you arrange things so that your URLs are inflected, you can always tell that a URL that begins /resource/new will be a particular resource, with the permalink ‘new’, and /resources/new will be the virtual new resource.

If the resource system is factored as I outlined, this is almost trivial, you can introduce a InflectedResource subclass of Resource

class InflectedResource < Resource
  def member_path
    @new_path ||= #{path_prefix}/#{singular}/:id
  end
end

and you’re pretty much done. Admittedly, something like that (plus a small amount of copy and paste) would work with the current system, but then we’re looking at 3 substantially identical methods in ActionController::Resources and if it wasn’t time to refactor before, it’d definitely be time to refactor then.

Conclusions

Reification shouldn’t be something you do every day, but nor should it be something you do once a flood. Take a look at some of your projects and some of the places where you’re using hashes. Are those really hashes, or would they benefit from having some behaviour of their own? You can track down stalled reification by looking for anaemic classes; classes which have a lot of accessors but very little behaviour. Once you’ve found an anaemic class, look for all the places that instances of it get used. Try moving some of the client code into methods on your anaemic class. Do that a few times and you’ll end up with a real object.

If you’re fussy about never putting HTML in your models, you could end up with a mediating builder/presenter object as well, but until you start wanting to render the same structured info in different formats, I’d suggest biting the bullet and living with HTML in the model as a lesser evil than structural code. Your mileage may vary.