Compare data from two different tables using codeigniter

[eluser]Unknown[/eluser]
I am new to codeigniter and I want to write this mySql query

Code:

SELECT COUNT(DISTINCT source) AS NumberOfEvaluations
FROM COMMENT , EVALUATION
WHERE evaluation_id = EVALUATION.id AND EVALUATION.department_id = '$department_id' ;

into codeigniter.
This is my try

Code:

$this->db->select('t1.count(DISTINCT(source))');  
   $this->db->from('comment t1');
   $this->db->join('evaluation t2', 't2.id = t1.evaluation_id', 'left');    

   $this->db->where();
  
$query=$this->db->get();  
   return $query->num_rows();