Codeigniter multiple database join

I am using codeigniter framework. I am not much experienced in this framework. I need help of you guys. I want to join two table present in two different database. In both the table I have one common column. I understand, first I have to create a separate database group inside the file database.php. I have created the group and am able to use ths group in my model separately. I have also loaded another group in my model "default group". Whenever, I am trying to use each group separately it's working without any issue. But I am struggling how to make join on two database using these two database group.

Now I want to join two tables of different database using these two separate group. But I am not sure where exactly I am doing the mistake.

This is my model file.

class Bar_graph extends CI_Model {

                public function __construct () {
                     parent::__construct();
                     $this->db=   $this->load->database('default', TRUE);//This is the default group
                     $this->db2 = $this->load->database('db2', TRUE); //This is the new group I have created
                 }
 //kalix2 and Asterik are my two different database               
               public function join_two_database ()
                {    
                      $cust_id=2;
                      $this->db->select('Kalix2.ph_Companies.CompanyName');
                      $this->db2->select_sum('Asterik.cdr.call_length_billable');
                      $this->db2->select('Asterik.cdr.calldate');
                      $this->db->where('Kalix2.ph_Companies.Cust_ID',$cust_id);
                      $this->db->from('Kalix2.ph_Companies');
                      $this->db2->group_by('Asterik.cdr.CompanyName');
                      $this->db->limit(5);
                      $this->db->join('Asterik.cdr','Kalix2.ph_Companies.CompanyName = Asterik.cdr.CompanyName','inner');
                      $query = $this->db->get();
                     if ($query->num_rows > 0) {
                     return $query-> result(); 
                    }
                }

回答1:

AFAIK, you can't join these in one statement. You only have one connection to a database at a time. You'd have to run a query on one database, run a query on the second database and then manipulate the results of both within PHP.

回答2:

function join_two_table() {

    $this->db->select("database1.tablename.first_name,database1.tablename.last_name,tablename.*");

    $this->db->from('database2.tablename');

    $this->db->join('database1.tablename', 'database2.tablename.user_id = database1.tablename.user_id');

    $this->db->where('database2.tablename.status', 'open');

    $this->db->order_by('database2.tablename.id', 'DESC');

    $query = $this->db->get();
    return $query->result();
}