gistfile1.txt

2.0.0-p247 :001 > Village.first.top_change
  Village Load (0.2ms)  SELECT "villages".* FROM "villages" ORDER BY "villages"."id" ASC LIMIT 1
   (0.2ms)  SELECT MAX(rank) AS max_id FROM "changes" INNER JOIN "conversations" ON "conversations"."change_id" = "changes"."id"
NoMethodError: undefined method `default_scoped?' for 3:Fixnum
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/relation/merger.rb:45:in `initialize'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/relation/spawn_methods.rb:44:in `new'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/relation/spawn_methods.rb:44:in `merge!'
	from /home/jragon/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/relation/spawn_methods.rb:33:in `merge'
	from /home/jragon/Code/Rails/OTIS/app/models/village.rb:10:in `top_change'
	from (irb):1
	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>'

Models.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 & Change.top
  end
end

class Change < ActiveRecord::Base
  has_many :conversations

  validates :name, presence: true

  def self.top
    with_rank.maximum("rank")
  end
   
  def self.with_rank
    joins(:conversations).select("changes.*, SUM(conversations.rank) as rank")
  end
end

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

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"