unable to update record

Well, there are a few issues, but I can't be sure I've found any of the issues you're specifically looking for without a description of what's wrong (e.g. what you expect vs. what actually happens).

To start, though, remove the form elements from your edit view. You should not open a form then use the form_open() method inside the form, and you should use either form_close() or the </form> tag at the end of the form, not both. Also, if you want to display the result of $this->db->last_query() on your view, you should pass it from the controller instead of calling it directly in the view (it might work in the view, but there are a number of reasons it might not, so it should not be done).

Next, when using $this->db->set() and $this->db->update(), there's no reason to call $this->db->set($data) if you're passing $data to $this->db->update()'s second parameter. So your update method could look like this:

PHP Code:

public function update($data, $old_roll_no)
{
    $this->db->where("roll_no", $old_roll_no);
    $this->db->update("stud", $data);
} 


or you could chain the db calls:

PHP Code:

$this->db->where("roll_no", $old_roll_no)->update("stud", $data); 


If you really prefer the set() syntax, just omit $data from the update() call:

PHP Code:

$this->db->set($data)->where("roll_no", $old_roll_no)->update("stud"); 


Hopefully, the tutorial will eventually get around to adding form validation and other considerations, because you still wouldn't want to start using this code in production.

Bonfire

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