Rails - Apr 2008 - Encapsulating multiple model logic and checking returns

Hi,
I posted the other day, but I don''t think I was clear enough, this
post tries to break down the essence of the problem :)

What is the best way to check what happened within a method call
whilst still encapsulating the logic within the object. For example, I
have an object method which calls upon other objects to get results
back. Depending on the result of that call, different actions need to
take place.

So, I have something like this:

class Cart
  has_one :postage_method

  attr_accessor :postage_updated

  def add(product)
    # do adding product
    calculate_postage
  end

  def remove(product)
    # do removal of product
    calculate_postage
  end

  private

    def calculate_postage
      rate = postage_method.find_rate_for_weight(weight)
      if rate.some_condition
        # update db
        self.postage_updated = true
      else
        # update db
      end
    end
end

class CartController
  before_filter :get_cart_from_db # into @cart

  def add
    cart = @cart.add(product_from_params)
    if cart.postage_updated
      # tell user postage was automatically updated
    else
      # do something else
    end
  end
end

To me, the problem with this approach, is that you need to repeat
these checks for every method where you are calling the method, which
soon adds up and is not DRY. So:

class CartController
  before_filter :get_cart_from_db # into @cart

  def add
    cart = @cart.add(product_from_params)
    if cart.postage_updated
      # tell user postage was automatically updated
    else
      # do something else
    end
  end

  def remove
    cart = @cart.remove(product_from_params)
    if cart.postage_updated
      # tell user postage was automatically updated
    else
      # do something else
    end
  end

  ..etc

end

Also, what happens if I need to check multiple different conditions?
Am I going about this in entirely the wrong way?

Many thanks for any advice.
Cheers,
Jordan
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