Rails Model View Controller + Presenter?

Ruby on Rails directs you to use Model View Controller by convention. This often results in a one to many relationship between Controllers and Views. A common controller could contain the following code.

class StandardsController < ApplicationController
  
  def list
    @standards = Standard.find(:all)
  end
  
  def results
    standards = Standard.find(:all)
    @total_standards = standards.size
    @standards_for_hr = standards.select { |standard| standard.department == 'hr }
    @standard_categories = standards.collect { |standard| standard.category }.uniq
    ...
  end
  
end

This controller implementation works fine; however, the results method can become a bear to test. To solve this issue my team began inserting another layer: a Presenter. The main responsibility of a Presenter is to expose data for the view to consume. After introducing the presenter the controller becomes much slimmer.

class StandardsController < ApplicationController
  
  def list
    @standards = Standard.find(:all)
  end
  
  def results
    @presenter = Standard::ResultsPresenter.new()
  end
  
end

The Standard::ResultsPresenter class handles aggregating all the required data.

class Standard::ResultsPresenter

  def total_standards
    standards.size
  end
  
  def standards_for_hr
    standards.select { |standard| standard.department == 'hr }
  end
  
  def standard_categories
    standards.collect { |standard| standard.category }.uniq
  end 
  
  def standards
    Standard.find(:all)
  end
  
end

After introducing this abstraction the Presenter can be tested in isolation.

class Standard::ResultsPresenterTest < Test::Unit::TestCase

  def test_total_standards
    Standard.expects(:find).with(:all).returns([1,2,3])
    assert_equal 3, Standard::ResultsPresenter.total_standards
  end
  
  def test_standards_for_hr
    standard_stubs = [stub(:department=>'hr'), stub(:department=>'other')]
    Standard.expects(:find).with(:all).returns(standard_stubs)
    assert_equal 1, Standard::ResultsPresenter.standards_for_hr.size
  end
  
  def test_standard_categories
    standard_stubs = [stub(:catagory=>'Ruby'), stub(:catagor=>'Ruby')]
    Standard.expects(:find).with(:all).returns(standard_stubs)
    assert_equal ['Ruby'], Standard::ResultsPresenter.standard_categories
  end 

end

We like this style because it's much cleaner than the previously required code that lived in the controller tests. However, there is overhead involved in generating this additional layer. Because of the overhead we generally don't add a presenter until we notice that testing has become painful in the controller test file.