How to validate one data in model

Hi I am beginner at Codeigniter but I have 2 years experience in Laravel. 
I want to validate on data in model.

Code:

protected $validationRules = [
  'username' => 'required|alpha_numeric|max_length[25]|min_length[7]|is_unique[users.username]',
  'email' => 'required|valid_email|max_length[255]|is_unique[users.email]',
  'password' => 'min_length[8]',
  'key' => 'min_length[6]',
  'ip' => 'max_length[45]|valid_ip',
  'browser' => 'required'
  ];

When I need to save only username and email data. It gives me a validate error message in form but when I give all columns data it's works perfectly.
How I fixed this problem.