How to test method retry on optimistic locking without getting an inifite loop?

Question

I'm using optimistic locking, and when there is a failure on updating my object, I aim to retry the query until it passes. My problem is that the rescue section is never used on the rspec tests.

The method I'm talking about is the following:

def create_transaction
  amount = -1 * cost_per_word * word_count
  transaction = Transaction.new(amount: amount, processor: :job_done)
  transaction.service = service
  transaction.job = self
  transaction.save!
rescue ActiveRecord::StaleObjectError
  sleep rand
  reload
  retry
end

The StaleObjectError is generated on transaction.save!, which have a before_create hook which edits other object (in this case, removing money from the object account).

To test it, I could, for example, stub Transaction.new to raise an StaleObjectError, however it would generate an infinite loop.

I also can't test with an approach like

p1 = Person.find(1)
p2 = Person.find(1)

p1.first_name = "Michael"
p1.save

p2.first_name = "should fail"
p2.save # Raises a ActiveRecord::StaleObjectError

Because I'm not testing the before_save hook, I'm testing the create_transaction method. (thanks to @screenmutt for the example)

How can I properly test the rescue section?

Why the stubbing doesn't seem correct? That's what I would do, more or less. — Sergio Tulentsev, Aug 20 '13 at 18:23
@SergioTulentsev I would rather find an approach where the error is really generated, if possible. What if my code never throws StableObjectError by itself? — fotanus, Aug 20 '13 at 18:26
These are two different tests: testing that the error can be raised and testing that it, if raised, is handled properly. — Sergio Tulentsev, Aug 20 '13 at 18:29
@SergioTulentsev so perhaps my question is how to test if the error can be raised? — fotanus, Aug 20 '13 at 18:34
@SergioTulentsev I can't stub because it would lead to an infinite loop. — fotanus, Aug 20 '13 at 18:47

Dan Grahn · Answer 1 · 2013-08-20T18:40:27.153

1

First, A StaleObjectError will not be raised when creating an object. It may be raised when editing. See the Docs

p1 = Person.find(1)
p2 = Person.find(1)

p1.first_name = "Michael"
p1.save

p2.first_name = "should fail"
p2.save # Raises a ActiveRecord::StaleObjectError

Testing the Rescue Block

That is what stubs are for, returning a specific value from a method which you may not normally see.

You could also stub save!

RSpec Simple Stub Documentation

edited Aug 20 '13 at 18:40

answered Aug 20 '13 at 18:25

Dan Grahn

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Thanks, the stale object error happens on other model, in a before_save hook on transaction model (the transaction removes or add money in the other model balance), so I can't use this straight-forward approach. Sorry, I didn't make it clear on the question, let me edit it. – fotanus Aug 20 '13 at 18:49
I don't think you are going to be able to time a `StaleObjectError` if it happens in a before save. Unless you could delay the method's save somehow. – Dan Grahn Aug 20 '13 at 18:52

score 0 · Accepted Answer · edited May 23 '17 at 12:14

0

The answer I need to be able to test is the unstub method, cited in this answer by Chris Heald

edited May 23 '17 at 12:14

Community

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answered Oct 12 '13 at 03:07

fotanus

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How to test method retry on optimistic locking without getting an inifite loop?

2 Answers2