change.rb

class Change < ActiveRecord::Base
  has_many :conversations

  validates :name, presence: true

  def self.top_ranked(load_conversation=true)
    with_rank(load_conversation).order("conversations_rank desc").limit(1)
  end
   
  def self.with_rank(load_conversation=true)
    q = select("changes.*, SUM(conversations.rank) as conversations_rank").group("changes.id")
    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

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

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(false)
  end
end