Implementing an Upvote System Question

Currently I'm in the process of extending the social network application with an upvote system, but I'm not quite sure how to fully implement this, and I was wondering if I can get some tips.

class Post(Model):
    timestamp = DateTimeField(default=datetime.datetime.now)
    user = ForeignKeyField(
        rel_model = User,
        related_name = 'posts'
    )
    name = TextField()
    content = TextField()
    upvotes = IntegerField(default=0)
    url = TextField()
    category = TextField()

    class Meta:
        database = DATABASE
        order_by = ('-timestamp',)

I added an upvote property to the Post class with a default value of 0 for each post. My first attempt included making a function that would update a posts upvote property given an id from a link, but that wouldn't have worked because when making the link, lets say you are already viewing the post, Jinja won't let you do -> <a href={{url_for('stream', post_id={{post.id}}}}

This simply won't work. If anyone also knows a strategy of how to detect if the user had already upvoted, please let me know.

-----------------------------------------------MAJOR EDIT---------------------------------------------------------- Ok so I figured out how to increment my posts upvote property, but now I am stuck on how to make sure the user doesn't upvote more than once, and to decrement the upvote property of the post if the user clicks the button again. Here is how I accomplished the upvoting thus far:

#stream.html
<div class="voting_bar">
        <a href="/vote/{{post.id}}"><img src="/static/img/upvote.png"></a>
        <p>{{post.upvotes}}</p>
      </div>

Here I give the user a link, (recall before this that there was a for loop that went through all the posts in our database). When the user clicks the link, I pass the posts's id to this route:

@app.route('/vote/<int:post_id>')
def upvote(post_id):
    posts = models.Post.select().where(models.Post.id == post_id)
    if posts.count() == 0:
        abort(404)
    post = models.Post.select().where(models.Post.id == post_id).get()
    query = models.Post.update(upvotes = (post.upvotes+1)).where(models.Post.id == post_id)
    query.execute()
    return redirect(url_for('index'))

Now the only problem is identifying the user, and making it so that if they press the button again, they will just simply remove the upvote (also need to avoid negative upvotes, *There will be no down vote system in this app)

Regards, Guled