Is there a way to override a method of an instance variable

Question

I have an instance variable in an active record class called hash_value. It's a serialized hash.

I want to display the hash as XML. Is it right to call hash_value.to_xml? Many nodes are numbers, and XML tags are not allowed to be only number (or start with a number).

I want to override the to_xml method of hash_value. I don't want to override on all hashes, just the hash that's in this record.

class ProductVersion < ActiveRecord::base
   serialize :hash_value, Hash

   def hash_value.to_xml
   end
end

I tried the answer here redefining a single ruby method on a single instance with a lambda but it doesn't seem to be applicable. I suspect because when I load the record, it creates a new hash_value object and thus the singleton adjustment on the original is moot.

Any thoughts would be great. I know I could write a function hash_value_to_xml, but I'd rather avoid doing something like that.

Thanks to the first comment, I came up with a solution. Not a good one, but one that works. I'd love to see if there's a better way, because this one smells a bit.

class MyHash < Hash
    def to_xml
      1/0 #to see if it's run. 
    end
  end
  def hash_value
    MyHash.new().merge(  attributes['hash_value'] );
  end

`hash_value` is not an instance variable, BTW. It's a method. — Sergio Tulentsev, Jun 27 '13 at 06:49
cool... that actually helps me suss it out. Not sure why I didn't remember that. — baash05, Jun 27 '13 at 06:55
why the down vote? I want to ask better questions, please tell me what about the question is bad. Not enough code? Not clear? — baash05, Jun 27 '13 at 23:47

score 1 · Accepted Answer · answered Jun 27 '13 at 06:58

1

I would personally go for hash_value_to_xml route. But since you insist, here's an idea that might work (haven't tested that)

class ProductVersion < ActiveRecord::base
   serialize :hash_value, Hash

   alias_method :old_hash_value, :hash_value

   def hash_value
     h = old_hash_value
     h.define_singleton_method(:to_xml) do
      # your custom xml logic here
     end
     h
   end
end

The idea is that you intercept value returned from hash_value and patch it on the fly.

answered Jun 27 '13 at 06:58

Sergio Tulentsev

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I didn't do the alias I just used super, but man that was cool, and exactly what I was looking for. I didn't want to go with the underscore version because I don't want my user to know they aren't getting the normal to_xml. Especially since the native to_xml returns invalid xml E – baash05 Jun 27 '13 at 07:09

Is there a way to override a method of an instance variable

1 Answers1