Question

I have a rails application that serves as an interface to a hybrid of data. Most of the information I require is retrieved from the command-line program using XML-RPC. Aside from this, I require some additional bit of data which I have no option but to store in a database. For this reason, I am having trouble figuring out what would be the best way to design the application.

I have overridden self.all and self.find(id) such that they rely on calls to super and then “enrich” the object by defining its instance variables to the appropriate data retrieved from the program using XML-RPC.

This all seems pretty convoluted though. For example, I imagine I have lost the ability to use the magic finders (find_by_x), and I don’t know if anything else will break as a result of this.

My question is if there is a more logical and sensible way of going on about doing this. That is, designing an application that depends on XML-RPC for its data for the most part, but also some data stored in a database.

I did read about after_find. Using this callback, I can implement the “object enriching” process and have it run anytime there is a found record. However, my method of retrieving data associated with an item is different than that of retrieving all item data. The way I do it for retrieving all item data (self.all) is way more efficient, but unfortunately not applicable, to retrieving only one item’s data (self.find). This would work well if there were a way I could make the callback not apply to self.all calls.

Answer 1

In my experience, you shouldn’t mess with ActiveRecord’s finders – there is a lot of magic that they rely on.

after_find is a great direction to start with, but if you’re having issues with batching, then what I’d recommend is twofold – use a caching layer and alias_method_chain to implement a version of #all that performs your batched XML-RPC find, caches it, and then pass the call through to the unaliased original all. Then, your after_find would check your cache for data first, and if it’s not there, perform the remote find. This would let you successfully batch data for all finds while utilizing the callback.

That said, there is probably an easier way to do this. I would just use models that don’t descend from ActiveRecord::Base, but rather, which descend from some XMLRPC base interface, and then have faux associations on them that point to AR instances with your database information. Thus, you might have something like:

class XmlRpcModelBase
  ...

  def find(...)
  end

  def all(...)
  end

  def extra_data
    @extra_data ||= SomeActiveRecordModel.find(...)
  end
end

class Foo < XmlRpcModelBase

end

It’s not ideal, and honestly, it’s going to depend a lot on how much of this is read, and how much is read/write, but I would try to stay out of ActiveRecord’s way where possible, and instead just bolt on the AR-related pieces as necessary.