Tuesday, April 21, 2009

Unit Test eager loading in Rails ActiveRecord association

To avoid the n+1 problem, you need to use eager loading in ActiveRecord object associations. An easy implementation of eager loading can be done using named scope in the following way.

class User < ActiveRecord::Base
has_many :friends
named_scope :eager_load_friends, :include => :fiends
To unit test the named scope, you can use this following helper method (keep it in test_helper.rb, if you like) that I wrote for ScrumPad
def test_eager_loaded(model, association)
assert !model.instance_variable_get("@#{association.to_s}").nil?
You can then test your eager loading in the following way
def test_eager_load_friends
test_eager_loaded(User.find(1), :friends)
You can also use the shoulda plug-in if you like. For me, I think we should test the values returned by our named scope as opposed to shoulda approach, which tests if the code is written as expected.

Got another solution? I will appreciate if you share.