Why doesn't this Ruby on Rails code work as I intend it to?

So I attempted to build what I asked about in this question: Fix voting mechanism

However, this solution doesn't work. A user can still vote however many times he or she wants. How could I fix this and/or refactor?

def create       
    @video = Video.find(params[:video_id])
    @vote = @video.video_votes.new
    @vote.user = current_user

    if params[:type] == "up"
      @vote.value = 1
    else
      @vote.value = -1
    end

  if @previous_vote.nil?
    if @vote.save
      respond_to do |format|
        format.html { redirect_to @video }
        format.js
      end
    else
      respond_to do |format|
        format.html { redirect_to @video }
        format.js {render 'fail_create.js.erb'}
      end
    end
  elsif @previous_vote.value == params[:type]
    @previous_vote.destroy
  else
    @previous_vote.destroy    
    if @vote.save
      respond_to do |format|
        format.html { redirect_to @video }
        format.js
      end
    else
      respond_to do |format|
        format.html { redirect_to @video }
        format.js {render 'fail_create.js.erb'}
      end
    end
  end
  @previous_vote = VideoVote.where(:video_id => params[:video_开发者_开发问答id], :user_id => current_user.id).first
end

@previous_vote seems to be nil at the beginning of every request?

I would personally do away with all the logic in the controller and put uniqueness contraints at Model or database level where they belong.

Updated

This is probably full of errors but treat it as pseudo-code

Models something like:

class Video < ActiveRecord::Base
  has_many :votes
end

class Vote < ActiveRecord::Base
  belongs_to :user
  belongs_to :video
  validates_uniqueness_of :user_id, :scope => :video_id # only one vote per person per video
end

class User < ActiveRecord::Base
  has_many :votes
end

Controller:

def create
  @video = Video.find(params[:video_id])
  @vote = current_user.votes.find_or_create_by_video_id(@video.id)

  # change this next block of code so you assign value to the vote based on whatever logic you need
  if you_need_to_do_anything_to_change_its_value
    @vote.value = :whatever
  end

  if @vote.save
    redirect_to @video
  else
    render :whatever_is_appropriate
  end
end