title: How codeigniter's is_unique[] work
body: <p>I have checked codeigniter's documentations but there is no special page or information about it. All I wanna know is how it's working. I have this code and it's working!, What I'm wondering how is_unique working when there's no access to  any model or database. Is there any class in codeigniter that is_unique uses it to retrieve data based on the information like the name of the table and the field you give it to?
  and that's the code:</p>
  
  <p>Email input in the view:</p>
  
  <pre><code>echo form_input('email_address', set_value('email_address'), 'placeholder="Email"');
  </code></pre>
  
  <p>And the controller:</p>
  
  <pre><code>function create_member()
      {
          $this-&gt;load-&gt;library('form_validation');
  
          // field name, error message, validation rules
          $this-&gt;form_validation-&gt;set_rules('email_address', 'Email Address', 'trim|required|valid_email|is_unique[membership.email]');
          $this-&gt;form_validation-&gt;set_rules('username', 'Username', 'trim|required|min_length[4]');
          $this-&gt;form_validation-&gt;set_rules('password', 'Password', 'trim|required|min_length[4]|max_length[32]');
          $this-&gt;form_validation-&gt;set_rules('password2', 'Password Confirmation', 'trim|required|matches[password]');
  
          $this-&gt;form_validation-&gt;set_error_delimiters('&lt;div class="alert alert-error"&gt;&lt;a class="close" data-dismiss="alert"&gt;×&lt;/a&gt;&lt;strong&gt;', '&lt;/strong&gt;&lt;/div&gt;');
  
          if($this-&gt;form_validation-&gt;run() == FALSE)
          {
              $this-&gt;load-&gt;view('pages/signup_form');
          }
  
          else
          {           
              $this-&gt;load-&gt;model('admin/Users_model');
  
              if($query = $this-&gt;Users_model-&gt;create_member())
              {
                  $this-&gt;load-&gt;view('pages/signup_successful');           
              }
              else
              {
                  $this-&gt;load-&gt;view('pages/signup_form');         
              }
          }
  
      }
  </code></pre>
  
  <p>and her's the model:</p>
  
  <pre><code>function create_member()
      {
  
              $new_member_insert_data = array(
                  'email' =&gt; $this-&gt;input-&gt;post('email_address'),         
                  'user_name' =&gt; $this-&gt;input-&gt;post('username'),
                  'pass_word' =&gt; md5($this-&gt;input-&gt;post('password'))                      
              );
              $insert = $this-&gt;db-&gt;insert('membership', $new_member_insert_data);
              return $insert;
  
      }
  </code></pre>