From: John Wilger <johnwilger@...>
Date: 2004-11-19T04:09:55+09:00
Subject: Needle/Rails, Injected - Interceptors - Attn: Jamis Buck

Jamis (or anyone else who cares to give it a go),

I'm having a bit of trouble wrapping my head around the usage of
interceptors with Needle. I've implemented the changes you outlined in
you code examples from you proposal on "Rails, Injected"
(http://ruby.jamisbuck.org/rails-injected.html) and would like to use
the method interception within a few of my ActiveRecord classes.
Specifically, I have the following (abbreviated) classes:

-- snip --

class Project < ActiveRecord::Base
  has_many :story_cards

  def status
    ProjectStatus::find(self.status_name)
  end

  def status=(new_status)
    if ProjectStatus.include?(new_status)
      self.status_name = new_status.name
    else
      raise ArgumentError, "Only predefined instances of ProjectStatus
can be assigned."
    end
  end
end

class StoryCard < ActiveRecord::Base
  belongs_to :project
end

-- end snip --

And I have RankedValue/ProjectStatus defined as:

-- snip --

module RankedValue
  include Comparable

  attr_reader :name, :rank
  
  def initialize(name, rank)
    @name = name
    @rank = rank
  end

  def self.included(mod)
    mod.module_eval <<-EOF,__FILE__,__LINE__
    def self.new(*args, &block)
      name = args[0]
      name = name.upcase.gsub(' ', '_')
      const_set(name, super(*args, &block))
    end
    private_class_method :new
    
    def self.to_a
      constants.collect! { |cnst| const_get(cnst) }
    end

    def self.include?(obj)
      self.to_a.include?(obj)
    end

    def self.find(name)
      begin
        name_trans = name.upcase.gsub(' ', '_')
        const_get(name_trans)
      rescue NameError
        raise "No predefined instance of #{mod} matches #name == '\#{name}'."
      end
    end
    EOF
  end

  def <=>(other)
    self.rank <=> other.rank
  end
end

class ProjectStatus
  include RankedValue

  def initialize(name, rank, closed)
    super(name, rank)
    @closed = closed
  end

  def is_closed?
    @closed
  end

  new('Pending', 1, false)
  new('In Progress', 2, false)
  new('On Hold', 3, true)
  new('Completed', 4, true)
  new('Cancelled', 5, true)
end

-- end snip --

So, basically, I have the following instances of ProjectStatus available:

* ProjectStatus::PENDING
* ProjectStatus::IN_PROGRESS
* ProjectStatus::ON_HOLD
* ProjectStatus::COMPLETED
* ProjectStatus::CANCELLED

each of which responds to #is_closed? with true or false depending on
how the instance was initialized.

OK, now that we're through all the background (whew!), here's what I
want to be able to do. I want to make sure that a StoryCard can only
be assigned to a Project when Project#status.is_closed? == false.

I know that I could implement this by overriding the
StoryCard#project_id= method, but this seems to break encapsulation.
It should ultimately be the Project class's responsibility to decide
if a StoryCard can be assigned to it. I'm thinking I could create an
interceptor within the Project class definition to intercept the calls
to StoryCard#project_id= and add the necessary advice before the
assignment takes place (and raise a RuntimeError if the Project is
closed), but I'm not sure where to start.

Any advice?

-- 
Regards,
John Wilger

-----------
Alice came to a fork in the road. "Which road do I take?" she asked.
"Where do you want to go?" responded the Cheshire cat.
"I don't know," Alice answered.
"Then," said the cat, "it doesn't matter."
- Lewis Carrol, Alice in Wonderland