Using Mysql WHERE IN clause in codeigniter

You can use sub query way of codeigniter to do this for this purpose you will have to hack codeigniter. like this
Go to system/database/DB_active_rec.php
Remove public or protected keyword from these functions

public function _compile_select($select_override = FALSE)
public function _reset_select()

Now subquery writing in available
And now here is your query with active record

$this->db->select('trans_id');
$this->db->from('myTable');
$this->db->where('code','B');
$subQuery = $this->db->_compile_select();

$this->db->_reset_select();
// And now your main query
$this->db->select("*");
$this->db->where_in("$subQuery");
$this->db->where('code !=', 'B');
$this->db->get('myTable');

And the thing is done. Cheers!!!
Note : While using sub queries you must use

$this->db->from('myTable')

instead of

$this->db->get('myTable')

which runs the query.
Watch this too

How can I rewrite this SQL into CodeIgniter’s Active Records?

Note : In Codeigntier 3 these functions are already public so you do not need to hack them.