Rspec tests failing randomly when analysing ActiveRecord objects generated by Mongoid events

Question

I implemented an activity logging mechanism based on Mongoid that saves events in MongoDB.

The Mongoid model Activity has after_create events that perform different tasks depending on the type of activity logged: (Simplified example)

class Activity
  include Mongoid::Document

  after_create do |activity|
    method_name = "after_#{activity.event_type}"
    send(method_name) if respond_to? method_name
  end

  def after_user_did_something
    MyItem.create!(:type => :user_did_something)
  end
end

The test looks like this:

 it 'should hide previous [objects] create a new updated one' do
      2.times do 
        user.log_activity(:user_did_something) 
      end
      items = MyItems.where(:type => :user_did_something)
      items.count.should == 2
    end
 end

Sometimes, the tests fails on items.count being 0 instead of 2. This happens only when running from the command line rspec spec it never happens when running only this test, or when running all the tests with Guard.

Answer 1

In a typical Mongodb setup, there can be a delay between when a database write returns successfully and when that data can be read. There are two reasons for this:

• For performance gains, an "unsafe" write can return before the data is committed to the disk.
• Mongodb uses replica sets and there is a replication delay. Commonly reads are distributed to the replicas as a form of load balancing, so even if you use a safe write, you may be reading from a different server than the one you just wrote to and thus not see the data you just wrote.

To ensure that you can always immediately read back the data you just wrote using Mongoid, you need to set the database session options consistency: :strong, safe: true, neither of which are the default.