fixtures_vs_factories.rb

# The controller; yeah it's not great but I want to show the tests
class ProductsController < ApplicationController
  def index
    if params[:min_rating]
      @products = Product.where("average_rating >= ?", params[:min_rating])
    else
      @products = Product.to_a
    end
  end
end

# The test with factories:
class ProductsControllerTest < ActiveSupport::TestCase
  test "searching with a minimum rating filter" do
    # Every controller test has to create its' data independently, or else
    # shove common FactoryGirl.create calls into shared test setup.
    #
    # This gets really bloated and slow, *really* fast.
    p1 = FactoryGirl.create :product, average_rating: 3
    p2 = FactoryGirl.create :product, average_rating: 4
    
    get :index, min_rating: 4
    
    assert_equal [p2], assigns(:products)
  end
end

# The test with fixtures:
class ProductsControllerTest < ActiveSupport::TestCase
  test "searching with a minimum rating filter" do
    # *Zero* ceremony behind test setup means I can spit out a ton
    # of controller tests with as much ease as I could unit tests.
    get :index, min_rating: 4
    assert_equal [p2], assigns(:products)
  end
end

# test/fixtures/products.yml
sweater:
  name: "Corduroy sweater"
  average_rating: 3
  cost: 45

scarf:
  name: "Purple scarf"
  average_rating: 4
  cost: 88

The key is understanding that the fixtures in test/fixtures/products.yml can (and should) be reused pervasively throughout your test suite. It is work to maintain your fixtures, but once you invest that effort, controllers -- and really all your application logic living above your models -- are a breeze to test.

By the way, the test that uses fixtures will be unbelievably fast. Persistence in your tests doesn't have to be slow. Factories, however, are extremely slow in cases where you have to hit the db. Fixtures run in a nice transaction and don't have to get rebuilt between tests.

Also, can you imagine what would happen if you didn't have an average_rating column on products? If you had to join against actual ratings rows? The fixtures approach would have no problem with it, but suddenly, life with factories got much harder.

A few fixtures per model can go a long way; when devs start creating new fixtures for every mutation, things get ugly fast. But those are bad fixture sets, not a problem with the feature itself.