title: or_where is not working in codeigniter model
body: <p>I have a query model.But the second or_where is not working</p>
  
  <p><strong>model</strong></p>
  
  <pre><code>    $this-&gt;db-&gt;select('st_student.st_id');
      $this-&gt;db-&gt;from('st_student'); 
      $this-&gt;db-&gt;where('st_status',1);
      $this-&gt;db-&gt;or_where('st_status',2);  
      if(($from!='') &amp;&amp; ($to!='')){  
          $this-&gt;db-&gt;where("ab_date BETWEEN '$from' AND '$to'");
          $this-&gt;db-&gt;or_where("as_date BETWEEN '$from' AND '$to'");
      } 
      $this-&gt;db-&gt;group_by('st_student.st_id');    
      $result=$this-&gt;db-&gt;get();
  </code></pre>
  
  <p><strong>sql query</strong></p>
  
  <pre><code>SELECT `st_student`.`st_id`
  FROM (`st_student`)
  WHERE `st_status` =  1
  OR `st_status` =  2
  AND `ab_date` BETWEEN '01/15/2016' AND '01/26/2016'
  AND `as_date` BETWEEN '01/15/2016' AND '01/26/2016'
  GROUP BY `st_student`.`st_id`
  </code></pre>
  
  <p>Whats wrong in that</p>