Joomla Comonent update data - php

Question

I'm working on a Joomla 3 component. Currently I'm programming the backend. I'm having a form to add new data and it is working quite well. But when I want to update the data the component creates a new item instead of updating the existing.
I was searching for position which let Joomla know, that this is an update, but without success.
So my question: what is the information that makes Joomla updating the data?
My Code:
Table:ia.php
class mkTableia extends JTable
{
/**
* Constructor
*
* #param object Database connector object
*/
function __construct(&$db)
{
parent::__construct('#__tbl_ia', 'ID', $db);
}
}
Model: ia.php
class mkModelia extends JModelAdmin
{
public function getTable($type = 'ia', $prefix = 'mkTable', $config = array())
{
return JTable::getInstance($type, $prefix, $config);
}
public function getForm($data = array(), $loadData = true)
{
// Get the form.
$form = $this->loadForm('com_mk.ia', 'ia',
array('control' => 'jform', 'load_data' => $loadData));
if (empty($form))
{
return false;
}
return $form;
}
protected function loadFormData()
{
// Check the session for previously entered form data.
$data = JFactory::getApplication()->getUserState('com_mk.edit.ia.data', array());
if (empty($data))
{
$data = $this->getItem();
}
return $data;
}
}
View:view.html.php
class mkViewia extends JViewLegacy
{
/**
* display method of Hello view
* #return void
*/
public function display($tpl = null)
{
// get the Data
$form = $this->get('Form');
$item = $this->get('Item');
// Check for errors.
if (count($errors = $this->get('Errors')))
{
JError::raiseError(500, implode('<br />', $errors));
return false;
}
// Assign the Data
$this->form = $form;
$this->item = $item;
// Set the toolbar
$this->addToolBar();
// Display the template
parent::display($tpl);
}
/**
* Setting the toolbar
*/
protected function addToolBar()
{
$input = JFactory::getApplication()->input;
$input->set('hidemainmenu', true);
$isNew = ($this->item->ID == 0);
JToolBarHelper::title($isNew ? JText::_('COM_MK_MANAGER_MK_NEW')
: JText::_('COM_MK_MANAGER_MK_EDIT'));
JToolBarHelper::save('IA.save');
JToolBarHelper::cancel('IA.cancel', $isNew ? 'JTOOLBAR_CANCEL'
: 'JTOOLBAR_CLOSE');
}
}