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# app/models/vote.rb
class Vote < ActiveRecord::Base
  belongs_to :post
  validates_presence_of :user_ip, :post
end

class Post < ActiveRecord::Base
  has_many :votes

  def voted_from?(ip)
    !self.votes.where(:user_ip => @ip).empty?
  end
end

# app/controllers/votes_controller.rb
class VotesController < ApplicationController
  def create
    ip = request.remote_ip
    post = Post.find(params[:id])

    unless post.voted_from?(params[@ip])
      Vote.create(params[:vote])
    end
  end
end