Insert New Row If Not Update - php

Question

When my user login on in the activity's model. If the user has logged on before it updates that row belonging to the user id. Otherwise should Insert row.
Problem: Currently it just updates the same row as previous. It should insert a new row if user id has not logged on before.
How can I fix problem on model.
<?php
class Model_account_activity extends CI_Model {
public function updateActivity($key, $data) {
$data['user_id'] = $this->session->userdata('user_id');
if (isset($data['user_id'])) {
$user_id = $data['user_id'];
} else {
$user_id = 0;
}
$query = $this->db->query("UPDATE `" . $this->db->dbprefix . "user_activity` SET
`user_id` = '" . (int)$user_id . "',
`key` = " . $this->db->escape($key) . ",
`data` = " . $this->db->escape(serialize($data)) . ",
`date_added` = NOW()
");
if ($query == FALSE) {
$this->addActivity();
return true;
} else {
return false;
}
}
public function addActivity($key, $data) {
$data['user_id'] = $this->session->userdata('user_id');
if (isset($data['user_id'])) {
$user_id = $data['user_id'];
} else {
$user_id = 0;
}
$this->db->query("INSERT INTO `" . $this->db->dbprefix . "user_activity` SET
`user_id` = '" . (int)$user_id . "',
`key` = " . $this->db->escape($key) . ",
`data` = " . $this->db->escape(serialize($data)) . ",
`date_added` = NOW()
");
}
}

Answer 1

Get result from DB by unique ID or primary key then check the query result. If result count is 0 then can insert else update. Example...
$this->db->where('column_name', $user_id); //Unique column or primary_key .It may be $user_id
$query = $this->db->get('table_name');
if($query->num_rows == 0)
{
//Insert query
}
else
{
//Update query
}

Answer 2

Here is how i was able to get it working thanks to #Please Wait all working now.
<?php
class Model_account_activity extends CI_Model {
public function activity($key, $data) {
$user_id = $this->session->userdata('user_id');
$this->db->where('user_id', $user_id);
$query = $this->db->get('user_activity');
$data['user_id'] = $this->session->userdata('user_id');
if (isset($data['user_id'])) {
$user_id = $data['user_id'];
} else {
$user_id = 0;
}
if($query->num_rows() == FALSE) {
$data = array(
'user_id' => $user_id,
'key' => $key,
'data' => serialize($data),
'date_added' => mdate('%Y-%m-%d %H:%i:%s', now())
);
$this->db->insert_id();
$this->db->insert('user_activity', $data);
} else {
$data = array(
'user_id' => $user_id,
'key' => $key,
'data' => serialize($data),
'date_added' => mdate('%Y-%m-%d %H:%i:%s', now())
);
$this->db->where('user_id', $user_id);
$this->db->update('user_activity', $data);
}
}
}