Single Table Inheritance and Active Record Observers
Active Record has a nice feature that enables the programmer to separate concerns. In the application I am building, I need to reject some transactions based on business rules. For example, if an attacker attemps to empty an account by requesting multiple payouts, or attempting to load more than a specific amount of money in the user’s account.
In this application, I am using STI (Single Table Inheritance) to represent the different types of transaction. The hierarchy looks like this:
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class GreenbackTransaction < ActiveRecord::Base end class AccountRechargeTransaction < GreenbackTransaction end class AccountPayoutTransaction < GreenbackTransaction end |
I defined my observer like this:
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class GreenbackTransactionObserver < ActiveRecord::Observer def after_create(txn) # Code to reject the transaction if it is suspicious end end |
Using the console, everything was working fine. So, off I went to write a test for it (I know, it should have been the other way around, but I haven’t used observers much, and I wanted to see what was going on).
My test is defined like this:
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class GreenbackTransactionObserverTest < Test::Unit::TestCase fixtures :greenback_transactions, :users, :accounts, :affected_accounts def setup @sam = users(:sam) Setting.recharge_amount_threshold = 150.to_money end def test_suspicious_transaction_rejected assert_nothing_raised do AccountRechargeTransaction.new(:account => @sam.account, :amount => 100.to_money) end assert_raise(TransactionFailureException) do AccountRechargeTransaction.new(:account => @sam.account, :amount => 100.to_money) end end end |
To my complete surprise, this didn’t work. After much investigation, I found that the observer was not loaded for GreenbackTransaction. After some fooling around, adding logging statements in Rails core, I finally stumbled upon the solution:
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class AccountRechargeTransactionObserver < ActiveRecord::Observer observe AccountRechargeTransaction def after_create(txn) # Code to reject the transaction if it is suspicious end end |
The problem was the observer was registered on GreenbackTransaction, and it seems the observers aren’t inherited in subclasses. This is important and bears repeating: if you use STI and observers, observe your subclasses !.
