changing object in loop in Datamapper ORM to save

[eluser]mak.gnu[/eluser]
When I run the following code it gives error stating

Code:

The id field must contain an integer.

Code:

$t = new Truck();
        foreach ($t->fields as $k => $val){
            $t->$val = $this->input->post("$val");
        }
        if (!$t->save()) {
            // or we can loop through the error's all list
            foreach ($t->error->all as $error) {
                echo $error;
            }
        }

And whenI run following code it works like a charm. and saves the data.

Code:

$t = new Truck();
        foreach ($_POST as $k => $val){
            $t->$k = $this->input->post($k);
        }
        if (!$t->save()) {
            // or we can loop through the error's all list
            foreach ($t->error->all as $error) {
                echo $error;
            }
        }

What is wrong in using $t->fields