Active Record => What if class variables don't match database table columns?

[eluser]momsenmeister[/eluser]
Hi,

my model classes look like this:

Code:

class User extends Model
{
  var $id = '';
  var $firstname = '';
  var $surname = '';

  function update_entry()
  {
    $this->firstname = $this->input->post('firstname');
    $this->surname = $this->input->post('surname');
    $this->db->where('id', $this->id);
    $this->db->update('user', $this);
  }
}

For updating data I use a function like update_entry().

Like this I can always just use

Code:

$this->user->firstname

in my views, for example.
But my problem is: If my class variables don't exactly match my database table columns, I can't use

Code:

$this->db->update('user', $this);

or I get an exception

Unknown column in field list.

I try to avoid building a specific array each time I want to update some data.
Is there any way to just ignore columns, that don't exist in my database table?
For example, I might have an extra variable called "initials" in my model to use

Code:

$this->user->initials

in my views, but I might not need an extra column in my database table for it.

Any solution? Any best practices?

Thx!