CodeIgniter 3 - Change select values depending on the other select

Question

I have a carro table that has a relationship with marca and modelo , and these two also have a relationship. I first add a marca , then when adding a new modelo , I must select a marca before.

Now, to add a new carro , I must select one marca and modelo . Currently all the registered modelo are being listed, but I need to list those related to the selected marca . Below is the select HTML for the fields.

<div class="row">
    <input id="hdn_id_marca" type="hidden" name="hdn_id_marca" value="0"/>
    <div class="form-group col-md-6">
        <label for="id_marca">Marca do Carro</label>
        <select id="id_marca" name="id_marca" class="form-control" required>
            <option value="" selected disabled>Selecione</option>
            <?php foreach ($marcas as $m) { ?>
                <option value="<?php echo $m['id']; ?>">
                    <?php echo $m['nome_marca']; ?>
                </option>
            <?php } ?>
        </select>
    </div>
    <div class="form-group col-md-6">
        <label for="id_modelo">Modelo do Carro</label>
        <select id="id_modelo" name="id_modelo" class="form-control" required>
            <option value="" selected disabled>Selecione</option>
            <?php foreach ($modelos as $m) { ?>
                <option value="<?php echo $m['id']; ?>">
                    <?php echo $m['nome_modelo']; ?>
                </option>
            <?php } ?>
        </select>
    </div>
</div>

These values are retrieved from the database, it is not any static value. I have tried to create an Ajax:

$('#id_marca').change(function() {
    var selectedId = $(this).find('option:selected').val();
    $('#hdn_id_marca').attr('value', selectedId);

    $.ajax({
        url: '/admin/modelos/select_by_id_marca',
        data: {'hdn_id_marca': selectedId},
        type: 'POST',
        success: function() {
            alert(data)
        }
    });
});

But I then I get

POST XHR http://localhost:8000/admin/modelos/select_by_id_marca [HTTP/1.1 500 Internal Server Error 53ms]

Edit 2

Below is my select_by_id_marca() method. I have also updated the previous code by adding the hidden field this method is suppose to have access to. After some changes, I fixed the reason for the 500 error to be happening, but now, the field is not populated only with the modelo according to the selected marca .

public function select_by_id_marca()
{
    $data = array();

    $this->db->order_by($this->primary_key, 'DESC');
    $this->db->get_where('marca', array('id' => $this->input->post('hdn_id_marca')));
    $query = $this->db->get('marca');

    foreach ($query->result_array() as $row) {
        $data[] = $row;
    }

    $query->free_result();

    return $data;
}

Answer 1

This is wrong

$this->db->order_by($this->primary_key, 'DESC');
$this->db->get_where('marca', array('id' => $this->input->post('hdn_id_marca')));
$query = $this->db->get('marca');

You are using both get_where() and get() . It should be

$this->db->order_by($this->primary_key, 'DESC');
$query = $this->db->get_where('marca', array('id' => $this->input->post('hdn_id_marca')));

Next,

change

return $data;

to

echo json_encode($data);

In ajax success,

success: function(data) {
    var a = JSON.parse(data);
    alert(a.id);
}