count all results codeigniter

$this->db
  ->where(['field'=>'foo'])
  ->from("tablename")
  ->count_all_results();

//OR 
$this->db
  ->where(['field'=>'foo'])
  ->count_all_results("tablename");

$this->db->select('id');
$this->db->from('table');
$this->db->where($your_conditions);
$num_results = $this->db->count_all_results();