Question

there is something that confuses me a little bit and i would like some clarification please, since it causes me some trouble.

I have a city model with a delegate to a wood_production attribute that specifies the amount of wood of that city. It’s like:

has_one :wood_production, :autosave => true
delegate :amount, :to => :wood_production, :prefix => true, :allow_nil => true

    def wood
        wood_production_amount
    end

    def wood= amt
      self[:wood_production_amount] = amt
    end

I normally wanted to be able to do a city.wood -= 1000 and save that value through the city, but i’ve come into all sorts of problems doings this. It seems that i am not setting my virtual attributes correctly maybe.

So i would actually like to ask, what is the difference between these :

def wood
    self.wood_production_amount
end

def wood
    wood_production_amount
end

def wood
    self[:wood_production_amount]
end

and what should really be used to correctly handle the situation ?

EDIT :

If i create the setter like :

def wood= amt
  self.wood_production_amount = amt
end

I get :

1.9.2p290 :003 > c.wood -= 1000
 => 58195.895014789254 
1.9.2p290 :004 > c.save
   (0.1ms)  BEGIN
   (0.3ms)  UPDATE `wood_productions` SET `amount` = 58195.895014789254, `updated_at` = '2012-01-24 02:13:00' WHERE `wood_productions`.`id` = 1
   (2.0ms)  COMMIT
 => true

1.9.2p290 :005 > c.wood
 => 66522.63434300483         ???????

Buf if the setter is :

def wood= amt
  wood_production_amount = amt
end

1.9.2p290 :004 > c.wood -= 1000
 => 58194.823000923556 
1.9.2p290 :005 > c.save
   (0.1ms)  BEGIN
   (0.2ms)  COMMIT
 => true

Answer 1

Answering the first part, self.wood_production_amount and wood_production_amount are functionally identical. The only difference is that in the latter, self is implied, being the current instance of the City model. I rarely use self.anything unless it’s required.

self[:wood_production_amount] is functionally similar to the first two in most cases. The difference is that it allows you to easily overwrite default accessor methods. read_attribute(:attribute) is functionally identical to self[:attribute]. For example, say your City model has a state attribute, but you want to always return the state in uppercase when it is requested. You could do something like this:

class City < ActiveRecord::Base
  def state
    self[:state].try(:upcase)
    # or
    read_attribute(:state).try(:upcase)
  end
end

city = City.new(:state => 'vermont')
city.state # => VERMONT

So to answer your second question, it really depends on how you want to use it. Personally, I would go with the delegate method unless you need to overwrite some behavior. The reason it wasn’t working for you might be that you aren’t delegating the setter method :amount= as well:

delegate :amount, :amount= :to => :wood_production, 
         :prefix => true, :allow_nil => true