[Fixed]-Dynamically assigning ID to an element with jQuery

this.id is undefind inside $.get. SO you should change your code like this

$(document).ready(function() {
      $('.vote_up').on('click', function() {
        var id = this.id;
        $.get("/vote_up/" + id + "/", function(data) {
          $('#voted' + id).html(data);
        });
      });
      $('.vote_down').on('click', function() {
        var id = this.id;
        $.get("/vote_down/" + id + "/", function(data) {
          $('#voted' + id).html(data);
        });
      });
    });