Active Record Reputation System

gistfile1.html

    -- <%= haiku.user.name %>
    | <%= pluralize haiku.votes, "vote" %>
    <% if current_user && current_user.can_vote_for?(haiku) %>
      | <%= link_to "up", vote_haiku_path(haiku, value: 1), method: "post" %>
      | <%= link_to "down", vote_haiku_path(haiku, value: -1), method: "post" %>
    <% end %>

haiku.rb

class Haiku < ActiveRecord::Base
  attr_accessible :content

  belongs_to :user
  has_many :haiku_votes

  def self.by_votes
    select('haikus.*, coalesce(value, 0) as votes').
    joins('left join haiku_votes on haiku_id=haikus.id').
    order('votes desc')
  end

  def votes
    read_attribute(:votes) || haiku_votes.sum(:value)
  end
end

haiku_vote.rb

class HaikuVote < ActiveRecord::Base
  attr_accessible :value, :haiku, :haiku_id

  belongs_to :haiku
  belongs_to :user

  validates_uniqueness_of :haiku_id, scope: :user_id
  validates_inclusion_of :value, in: [1, -1]
  validate :ensure_not_author

  def ensure_not_author
    errors.add :user_id, "is the author of the haiku" if haiku.user_id == user_id
  end
end

haikus_controller.rb

class HaikusController < ApplicationController
  before_filter :authorize, except: [:index, :show]

  def index
    @haikus = Haiku.by_votes
  end

  def show
    @haiku = Haiku.find(params[:id])
  end

  def create
    @haiku = current_user.haikus.create!(params[:haiku])
    redirect_to @haiku, notice: "Successfully created haiku."
  end

  def vote
    vote = current_user.haiku_votes.new(value: params[:value], haiku_id: params[:id])
    if vote.save
      redirect_to :back, notice: "Thank you for voting."
    else
      redirect_to :back, alert: "Unablet to vote, perhaps you already did."
    end
  end
end

routers.rb

resources :haikus do
  member { post :vote }
end

user.rb

class User < ActiveRecord::Base

  has_many :haikus
  has_many :haiku_votes

  def total_votes
    HaikuVote.joins(:haiku).where(haikus: {user_id: self.id}).sum('value')
  end

  def can_vote_for?(haiku)
    haiku_votes.build(value: 1, haiku: haiku).valid?
  end
end

http://railscasts.com/episodes/364-active-record-reputation-system?view=asciicast