check.js

  // JSON Array of Towns.. In Rails: <%= @towns = Town.all.to_json %>
  var TOWNS = [
    { town: "Abbeville city", id: 1 },
    { town: "Adamsville city", id: 2 },
    { town: "Addison Town", id: 3 }
  ]; 


$(document).ready(function() { 
  $('#postcard_city').bind('change', updateStateValue);
  
  // Function takes a town string and returns a town id from the TOWNS json array.
  var findTown = function(townName) {
    var townIdToReturn = false;
    for (var i = 0; i < TOWNS.length; i++) {
      if (TOWNS[i].town == townName) {
        townIdToReturn = TOWNS[i].id;
        break;
      } 
    }
    return townIdToReturn;
  }
  
  // the function that gets called on the bind event... 
  var updateStateValue = function(event) {
     var town = $('#postcard_city').val();
     var id = findTown(town);
     if (!id) {
       $('#hidden_town_id').html("");
     } else {
       $("#hidden_town_id").html("<input id='postcard_town_id' name='postcard[town_id]' type='hidden' value='" + id + "'>");
     }
  }

});

thanks bro