Change.all.each { |c| c.with_rank.rank }

2.0.0-p247 :004 > Change.all.each { |c| c.with_rank.rank }
  Change Load (0.6ms)  SELECT "changes".* FROM "changes"
NoMethodError: undefined method `with_rank' for #<Change:0x00000004272050>
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activemodel-4.0.0/lib/active_model/attribute_methods.rb:436:in `method_missing'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/attribute_methods.rb:131:in `method_missing'
	from (irb):4:in `block in irb_binding'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/relation/delegation.rb:13:in `each'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/relation/delegation.rb:13:in `each'
	from (irb):4
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0/lib/rails/commands/console.rb:90:in `start'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0/lib/rails/commands/console.rb:9:in `start'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0/lib/rails/commands.rb:64:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'
2.0.0-p247 :005 > 

change.rb

class Change < ActiveRecord::Base
  has_many :conversations

  validates :name, presence: true

  # def self.top
  #   with_rank.order("rank desc").limit(1)
  # end
   
  # def self.with_rank
  #   joins(:conversations).select("changes.*, SUM(conversations.rank) as rank")
  # end

  def self.top_ranked(load_conversation=true)
    with_rank(load_conversation).order("rank desc").limit(1)
  end
   
  def self.with_rank(load_conversation=true)
    q = select("changes.*, SUM(conversations.rank) as rank")
    q = q.joins(:conversations) if load_conversation
    return q
  end
end

conversation.rb

class Conversation < ActiveRecord::Base
  belongs_to :discussion
  belongs_to :change

  validates :discussion_id, :change_id, :rank, :ten_seed, presence: true
  validates :change_id, :rank, uniqueness: { scope: :dicussion_id } 
end

Conversations

2.0.0-p247 :002 > Conversation.all
  Conversation Load (0.8ms)  SELECT "conversations".* FROM "conversations"
 => #<ActiveRecord::Relation [
#<Conversation id: 1, discussion_id: 1, change_id: 2, rank: 1, ten_seed: 10, observations: "It helped\nA lot", created_at: "2013-11-21 19:28:09", updated_at: "2013-11-21 19:28:09">, 
#<Conversation id: 2, discussion_id: 1, change_id: 1, rank: 2, ten_seed: 8, observations: "It helped quite a bit", created_at: "2013-11-21 19:29:56", updated_at: "2013-11-21 19:29:56">, 
#<Conversation id: 3, discussion_id: 1, change_id: 4, rank: 3, ten_seed: 8, observations: "Wasn't the best", created_at: "2013-11-21 19:30:17", updated_at: "2013-11-21 21:34:51">]> 

2.0.0-p247 :003 > Change.all
  Change Load (0.3ms)  SELECT "changes".* FROM "changes"
 => #<ActiveRecord::Relation [
#<Change id: 1, name: "Health", created_at: "2013-11-21 19:19:44", updated_at: "2013-11-21 19:19:44">, 
#<Change id: 2, name: "School", created_at: "2013-11-21 19:19:49", updated_at: "2013-11-21 19:19:49">, 
#<Change id: 4, name: "Boarhole", created_at: "2013-11-21 19:20:28", updated_at: "2013-11-21 19:20:28">]> 

discussion.rb

class Discussion < ActiveRecord::Base
  belongs_to :village
  belongs_to :group
  has_many :conversations

  validates :lead_facilitator, :duration, :date_held, 
            :group_id, :village_id, presence: true
end

top_change.sql

2.0.0-p247 :004 > Village.first.top_change.to_sql
  Village Load (0.7ms)  SELECT "villages".* FROM "villages" ORDER BY "villages"."id" ASC LIMIT 1
 => "SELECT  changes.*, SUM(conversations.rank) as rank FROM \"changes\" INNER JOIN \"conversations\" \"conversations_changes\" ON \"conversations_changes\".\"change_id\" = \"changes\".\"id\" INNER JOIN \"conversations\" ON \"changes\".\"id\" = \"conversations\".\"change_id\" INNER JOIN \"discussions\" ON \"conversations\".\"discussion_id\" = \"discussions\".\"id\" WHERE \"discussions\".\"village_id\" = ?  ORDER BY rank desc LIMIT 1" 

village.rb

class Village < ActiveRecord::Base
  belongs_to :programme
  has_many :discussions
  has_many :conversations, through: :discussions
  has_many :changes, through: :conversations

  validates :name, :programme_id, presence: true
  
  def top_change
    changes.merge Change.top_ranked
  end
end