how to check if username already exists in codeigniter

Question

I am working on a site in codeigniter.I am not so expert using framework.Here I have to check if email already exists in database.I have coded the required functionality but

Answer 1

Use is_unique[table.field] add table name and field.

$this->form_validation->set_rules(
    'email', 'Email', 'trim|required|valid_email|is_unique[table.field]|callback_isEmailExist'
);