structure.rb

class Village < AR::Base
  has_many :discussions
  has_many :conversations, through: :discussions
  has_many :changes, through: :conversations
  
  def top_change
    changes.merge Change.top_ranked(false)
  end

end

class Discussion < AR::Base
  belongs_to :village
  has_many :conversations
end

class Conversation < AR::Base
  belongs_to :discussion
  belongs_to :change
end

class Change
  has_one :conversation
  
  def self.top_ranked(load_conversation=true)
    with_rank(load_conversation).order("rank asc").limit(1)
  end

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