Updating database based on checkbox Codeigniter - php

Question

I'm trying to update a database column using a checkbox. The idea is that I want to only display certain things on the homepage, so the checkbox should send either 0 or 1 to the DB. If the record has 0 in the first page column, it shouldn't appear on the homepage, if it has 1 it should appear.
I tried setting something up, but it doesn't work as intended (it just refreshes the page with no effect)
The checkbox:
<?php echo form_checkbox(array('name' => 'first_page', 'class'=>'checkbox-inline', 'value' => 1)); ?>
Controller:
if(null != ($this->input->post('first_page'))) {
$first_page = 1;
$book_id= $this->input->post('id'); (the id is sent to the controller via an extra hidden input field)
$this->book_model->first_page($first_page, $book_id);
}
Model:
$this->db->set ( "first_page", $first_page);
$this->db->where ( "book_id", $book_id);
$this->db->update ( "books" );
return;
Full form:
<?php echo form_open('admin/modifica_autor'); ?>
<?php echo form_hidden('id', $item->id_carte); ?>
<?php echo form_hidden('id_autor', $item->id_autor)?>
<label for="titlu_nou">Titlu:</label>
<?php echo form_input(array('name' => 'titlu_nou_carte', 'class'=>'form-control', 'value' => $item->titlu)); ?> <br>
<label for="autor_nou">Autor:</label>
<select name='autor_nous' class="form-control"><?php
foreach ( $autori2 as $row ) {
echo '<option value="' . $row->id_autor . '">' . $row->nume_autor . '</option>'; }
?></select><br>
<label for="domeniu_nou">Domeniu:</label>
<select name='domeniu_nou' class="form-control"><?php
foreach ( $domenii as $row ) {
echo '<option value="' . $row->id_domeniu . '">' . $row->nume_domeniu . '</option>';
}
?></select><br>
<label for="pret_nou">Pret:</label>
<?php echo form_input(array('name' => 'pret_nou', 'class'=>'form-control', 'value' => $item->pret.' LEI')); ?><br>
<label for="descriere_noua">Descriere:</label>
<?php echo form_textarea(array('name' => 'descriere_noua', 'class'=>'form-control textarea', 'value' => $item->descriere)); ?><br>
<label for="prima_pagina">Afisare pe prima pagina:</label>
<?php echo form_checkbox(array('name' => 'prima_pagina', 'class'=>'checkbox-inline', 'value' => 1)); ?>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
<?php echo form_submit(array('name'=>'submit_autor','value' => 'Modifica', 'class'=>'btn btn-default')); ?>
<?php echo form_close(); ?>