gistfile1.js

$(document).ready(function(){

		//Inicializando variables del select
		var parentNode =  $('select#t_comprobacion').val();
		var eventRun = false;
		
		
		$('select#t_comprobacion').change(function (){
			if(!eventRun  && parentNode != $('select#t_comprobacion').val()){
				$.getJSON("/gastos/ajax/json_tipos/"+$(this).val(), function(j){
      				var options = '<option>Todos los gastos</option>';
      				for (var i = 0; i < j.length; i++) {
        				options += '<option value="' + j[i].ID_DETALLES_GASTO + '">' + j[i].DESCRIPCION_GASTO + '</option>';
      				}
      				$("select#gastos").html(options);
					eventRun = true;
    			});
			}
		});
});