overwriting association accesor

rubyonrails-talk
1

Hi, first of all I would like to wish a happy new year to all the list members.

Here is my problem. I have a model class where I need to overwrite an accessor method in order to add some extra behaviour:

class LineItem < ActiveRecord::Base

  belongs_to :item, :polymorphic => true

  # add_item

2

Borja Martín wrote:

Hi, first of all I would like to wish a happy new year to all the list members.

Thank you Borja.

Here is my problem. I have a model class where I need to overwrite an accessor method in order to add some extra behaviour:

Look here: http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/4fe057494c6e23e8

3

Thanks Mark, I solved it by using the alias_method_chain as you suggested:

class LineItem < ActiveRecord::Base

  belongs_to :item, :polymorphic => true

  def item_with_price_check=(item)     self.item_without_price_check = item     self.price = item.price   end

  alias_method_chain :item=, :price_check

end

Regards

Mark Reginald James escribió:

4

Borja Martín wrote:

Hi, first of all I would like to wish a happy new year to all the list members.

Here is my problem. I have a model class where I need to overwrite an accessor method in order to add some extra behaviour:

class LineItem < ActiveRecord::Base

  belongs_to :item, :polymorphic => true

  # add_item   #   def item=(item)     write_attribute(:item, item)     self.price = item.price   end

  # item   #   def item     read_attribute(:item)   end end

What you need are association callbacks. The following is from the ActiveRecord::Association::ClassMethods Page in the RDoc:

Association callbacks

Similiar to the normal callbacks that hook into the lifecycle of an Active Record object, you can also define callbacks that get trigged when you add an object to or removing an object from a association collection. Example:

  class Project     has_and_belongs_to_many :developers, :after_add => :evaluate_velocity

    def evaluate_velocity(developer)       ...     end   end

It‘s possible to stack callbacks by passing them as an array. Example:

  class Project     has_and_belongs_to_many :developers, :after_add => [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]   end

Possible callbacks are: before_add, after_add, before_remove and after_remove.

Should any of the before_add callbacks throw an exception, the object does not get added to the collection. Same with the before_remove callbacks, if an exception is thrown the object doesn‘t get removed.