def to_representation(self, instance):
        ret = OrderedDict()
        fields = [field for field in self.fields.values() if not field.write_only]
        model = {}
        head = []
        for field in fields:
            try:
                key = field.get_attribute(instance)
            except SkipField:
                continue
            #
            if field.field_name != 'id':
                value = field.to_representation(key)
                # si es un update traemos el valor de la variable
                if instance.pk:
                    model.update({field.field_name: value['model']})
                    # excluimos al field activo
                    if field.field_name != 'activo':
                        head.append(value)
                else:
                    head.append(value)
                    if field.field_name == 'activo':
                        model.update({field.field_name: False})
                    else:
                        model.update({field.field_name: None})
        # agregamos el detalle al model
        ret.update({'title': instance._meta.verbose_name})
        ret.update({'model': model})
        # formulario sin detalles
        ret.update({'modelDetailRequired': False})
        ret.update({'cabecera': head})
        return ret