Query - Call to undefined method CodeIgniter\Database\MySQLi\Connection::where()

I get this error "Call to undefined method CodeIgniter\Database\MySQLi\Connection::where()" when I try this in my model class

PHP Code:

$this->db->table($table);
        $this->db->where($where);
        $this->db->update($save_data); 


however it works fine when

PHP Code:

$this->db->table($table)->where($where)->update($save_data); 


The first approach above worked fine in CI 3. So I suppose in CI 4 I just to have chain or save $db = $this->db;

A great explanation of Lonnie about this question.