Creating the model

The Urls_model contains three functions; obviously it contains our __construct() function but we're not focusing on that at the moment as it's not doing anything except referencing its parent.

Instead, let's look at the two functions save_url() and fetch_url(). As their names suggest, one saves information to the database and the other fetches information from it. For now, let's go and create the code and we'll discuss in detail what each function does later: Create the urls_model.php model file and add the following code to it:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Urls_model extends CI_Model {
  function __construct() {
    parent::__construct();
  }

  function save_url($data) {
    /*
    Let's see if the unique code already exists in 
    the database.  If it does exist then make a new 
    one and we'll check if that exists too.  
    Keep making new ones until it's unique.  
    When we make one that's unique, use it for our url 
    */
    do {
      $url_code = random_string('alnum', 8); 

      $this->db->where('url_code = ', $url_code);
      $this->db->from('urls');
      $num = $this->db->count_all_results();
    } while ($num >= 1);

    $query = "INSERT INTO `urls` (`url_code`, `url_address`) VALUES (?,?) ";
    $result = $this->db->query($query, array($url_code, $data['url_address']));

    if ($result) {
      return $url_code;
    } else {
      return false;
    }
  }

  function fetch_url($url_code) {
    $query = "SELECT * FROM `urls` WHERE `url_code` = ? ";
    $result = $this->db->query($query, array($url_code));
    if ($result) {
      return $result;
    } else {
      return false;
    }
  }
}

Let's take a look at save_url(). Notice the PHP construct do...while; it looks something like the following:

do {
// something
} while ('…a condition is not met');

So that means do something while a condition is not met.

Now, with that in mind, think about our problem. We have to associate the URL that the user has entered in the form with a unique value. We will use this unique value to represent the real URL.

Now there's no point using a sequential number (1, 2, 3, … 1000) as our unique value as someone can come along and iterate up through the numbers and get access to everyone's URLs. This may not be such a dreadful security risk as presumably all pages are accessible from the Internet anyway, but it's still not a good idea. So the unique value must not only be unique, it must be random and not easily guessed by passersby. Also, this unique value must only exist once in the database.

To ensure this, we will have to check if the unique value already exists and, if it does exist, make a new unique code and check in the database again.

So, let's look at the do while construct in the save_url() function in a bit more detail. The following is the code:

do {
            $url_code = random_string('alnum', 8); 

            $this->db->where('url_code = ', $url_code);
            $this->db->from('urls');
            $num = $this->db->count_all_results();
        } while ($num>= 1);

We use CodeIgniter's String helper and its random_string() function (make sure you include the String helper using $this->load->helper('string'); in your controllers' constructor). The random_string() function will create (as the name suggests) a random string of characters that we will use for our unique code.

In this case, we're asking random_string() to give us a string of characters made up of numbers and uppercase and lowercase letters; that string should be no more that 8 digits in length.

We then look into the database to see if the code random_string() has made for us already exists. We'll use the $this->db->count_all_results(); CodeIgniter function to count up the number of matching results.

If the unique string already exists, then the number returned by $this->db->count_all_results(); will be equal to 1 (as it already exists). If this happens, we will loop back to the beginning of the do while construct and start again by generating a new code.

We keep doing this until we find a code that does not exist in the database. When we do, we break out of the do while loop and save that unique code, along with the original URL to the database.

Now let's look at fetch_url(). We want to see if there is a record in the database that corresponds to the $url_code entered by the user (in this case, they have clicked on a URL). The fetch_url() function accepts $url_code as a function argument passed to it by the controller and looks for it in the database. If it is found, the entire record (table row) is returned to the controller; if not, it returns false. The controller handles the false result accordingly (it displays an error).