Why find(:last) fail in my unit tests?

Question

I have a one-to many relationship in Rails:

class User < ActiveRecord::Base
  has_many :activities, :order => "added_at DESC"


class Activity < ActiveRecord::Base
  belongs_to :user

I have a method in Activity:

def self.test_message(user, message)
  user.activities << Activity.create do |activity|
    activity.message = message
    activity.added_at = Time.now
  end    
end

and the following unit test:

require 'test_helper'

class ActivityTest < ActiveSupport::TestCase

  def test_test_message
    #From fixture
    alice = User.find_by_name("alice")
    assert_equal 0, alice.activities.count

    Activity.test_message(alice, "Hello")
    assert_equal 1, alice.activities.count

    Activity.test_message(alice, "Goodbye")
    assert_equal 2, alice.activities.count
    assert_equal "Hello", alice.activities.find(:first).message

    #The following line fails with: Goodbye expected but was Hello
    assert_equal "Goodbye", alice.activities.find(:last).message,
    acts = alice.activities
    assert_equal 2, acts.count
    assert_equal "Goodbye", acts[1].message
  end
end

which fails on the indicated line, but I can't work out why.

Also, using activities.find(:last) works when using the development environment, but only fails under the test environment. I have dropped and rebuilt the database.

Answer 1

This seems to be an issue with using the :order flag in your association declaration. This post isn't the exact situation you're experiencing, but it recommends against the practice in general:

http://weblog.jamisbuck.org/2007/1/18/activerecord-association-scoping-pitfalls

(I'm not sure if these suggestions are still relevant, but I was seeing the same behavior as you in Rails 2.3.3 until I made the changes below.)

I set up your app locally and tried applying the technique from comment #4 by adding

def Activity.by_added_at
  find :all, :order => 'added_at DESC'
end

and changing your find(:first) and find(:last) in the test to .by_added_at.first and .by_added_at.last, which returns more stable results.

One other suggestion -- your test is pretty big right now. You might consider splitting it out into multiple tests, each of which tests one or two conditions at most.