Alex Vollmer

I've just spent the last two hours pulling my hair out trying to get Single-Table Inheritance (STI) working with associations in ActiveRecord. After essentially walking through all of the possible ActiveRecord options in this setup, I finally stumbled upon a configuration that seems to work. So this post is an attempt to help the next poor bastard who is Googling in earnest for a solution to a similar problem.

So let's start with the domain model. I'm too spent at this point in the evening to port this to one of the standard examples. Instead I'll expose you to the domain of my particular problem. The app I'm working on is one that tracks (non-financial) lending transactions between two individuals. The parties involved, the item in question and when it's due are all tracked in the Transaction model (and transactions table). A Transaction has a number of states it walks through, using the acts_as_state_machine plugin. These state transitions are triggered by opaque-looking URLs that are sent via email to either party. These are one-time use actions that once consumed are no longer available. When an Action instance is created it also has a before_save callback that generates a unique ID (used in the URL) using Digest::SHA1.

So my plan was to have my Transaction class write one or more Action records for each possible action based on my state transitions. Take a look at the state diagram below:

I want to encapsulate the actual work to be performed within the Action instance the user invokes by following the link in their email. So my plan is to use STI to have different sub-classes of Action that operate on a transaction and march it forward to its next state polymorphically.

Now STI may appear to be total overkill for this problem, but here are my reasons for going this route:

I want to have these opaque IDs written down somewhere to associate a specific action with a URL
When the action is complete, I want to remove the record so it can't be performed again
The state for a given Transaction can have more than one possible action. I want a separate for each action.

Whew. Okay, clear so far? So my initial code looked something like this:

require "digest/sha1"

class Action < ActiveRecord::Base

  belongs_to :transaction
  before_save :create_guid

  def create_guid
    sha1 = Digest::SHA1.new
    sha1.update transaction_id.to_s
    sha1.update type.downcase
    sha1.update DateTime.to_s
    self.guid = sha1.hexdigest
  end
end

class ReturnAction < Action
  def execute
    transaction.return!
  end
end

class AbortAction < Action
  def execute
    transaction.abort!
  end
end

class DisputeAction < Action
  def execute
    transaction.abort!
  end
end

It seemed like a good idea at the time, but the strange thing was that no matter which incantation I tried, I simply couldn't create a new Action instance and have it write a record to the database. This simply didn't work:

ReturnAction.create! :transaction_id => 1

There were no errors on the returned object. No exceptions were thrown. No queries to the database and certainly no insert statements executed. Just complete and utter silence. Out of desperation, as much as anything else, I removed the belongs_to declaration from the Action class and instead declared a has_many on the Transaction class. Voila! It worked like a champ.

After a bit of thought, the has_many association makes complete sense to me in the case where we want to create new Action instances for a particular Transaction. However, if you look in the code above, the execute methods of each sub-class are referring to a transaction object/method—which I no longer have. However I don't necessarily need the full-blown belongs_to association here. I can just fake the bits I want in the parent Action class like so:

class Action < ActiveRecord::Base
  def transaction
    @transaction ||= Transaction.find(self.transaction_id)
  end
end

So none if this is particularly earth-shattering. Sorry folks, no great gems of philosophical wisdom today. Just one man's small accomplishment blown completely out of proportion.

ActiveRecord Fun Thay May Stump Only Me comments

Alex
Vollmer