change.rb

class Change < ActiveRecord::Base
  before_validation { |change| change.name.capitalize! }

  has_many :conversations, dependent: :destroy
  has_many :discussions, through: :conversations 
  has_many :groups, through: :discussions
   
  validates :name, presence: true, uniqueness: true


   
  def self.top_ranked(options = {})
    top.limit(1).first
  end

  def self.top(options = {})
    with_rank(options).order("score desc")
  end

  def self.with_rank(options = {})
    select("changes.*, ROUND((AVG(11 - conversations.rank) + COUNT(conversations.id)), 1) as score").group(options[:group_by] ? options[:group_by] : "changes.id").joins(:conversations)
  end

  def self.with_ten_seed(options = {})
    select("changes.*, ROUND(AVG(conversations.ten_seed), 1) as average_ten_seed").group("changes.id").joins(:conversations)
  end
end

development.log

Started GET "/national_offices" for 127.0.0.1 at 2013-12-17 16:52:37 +0000
Processing by NationalOfficesController#index as HTML
  NationalOffice Load (0.8ms)  SELECT "national_offices".* FROM "national_offices"
   (0.6ms)  SELECT COUNT(*) FROM "programmes" WHERE "programmes"."national_office_id" = $1  [["national_office_id", 1]]
  Rendered national_offices/index.html.erb within layouts/application (10.9ms)
Completed 500 Internal Server Error in 14ms

ActionView::Template::Error (undefined method `[]' for false:FalseClass):
    13:       <tr>
    14:         <td><%= link_to national_office.name, national_office %></td>
    15:         <td><%= national_office.programmes.count %></td>
    16:         <td><%= national_office.top_change.respond_to?(:name) ? national_office.top_change.name : "N/A" %></td>
    17:       </tr>
    18:     <% end %>
    19:   </tbody>
  app/models/change.rb:21:in `with_rank'
  app/models/change.rb:17:in `top'
  app/models/national_office.rb:10:in `top_change'
  app/views/national_offices/index.html.erb:16:in `block in _app_views_national_offices_index_html_erb___842741853516102737_69950254428600'
  app/views/national_offices/index.html.erb:12:in `_app_views_national_offices_index_html_erb___842741853516102737_69950254428600'


  Rendered /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/actionpack-4.0.0/lib/action_dispatch/middleware/templates/rescues/_trace.erb (1.5ms)
  Rendered /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/actionpack-4.0.0/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (1.5ms)
  Rendered /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/actionpack-4.0.0/lib/action_dispatch/middleware/templates/rescues/template_error.erb within rescues/layout (12.2ms)

index.html.erb

<p class="guidance">National Offices</h3>
<table>
  <thead>
    <tr>
      <th>Name</th>
      <th>Number of Programmes</th>
      <th>Top Change</th>
    </tr>
  </thead>

  <tbody>
    <% @national_offices.each do |national_office| %>
      <tr>
        <td><%= link_to national_office.name, national_office %></td>
        <td><%= national_office.programmes.count %></td>
        <td><%= national_office.top_change.respond_to?(:name) ? national_office.top_change.name : "N/A" %></td>
      </tr>
    <% end %>
  </tbody>
</table>

national_office.rb

class NationalOffice < ActiveRecord::Base
  belongs_to :regional_office
  has_many :programmes, dependent: :destroy
  has_many :groups, through: :programmes
  has_many :changes, through: :programmes

  validates :name, :regional_office_id, presence: true

  def top_change
    changes.merge(Change.top(false)).take
  end

  def changes_with_score
    changes.merge Change.with_ten_seed.top
  end
end