If you have same issue, undefined property $db in codeigniter 3 application then you are a right place and i can help to fix this issue quick.
If you used model and then you are directly going to use "$this->db" for access database table data. You will find this error "undefined property $db in codeigniter" If you are using codeigniter 3 project.
However, we can resolve this issue in just a sec. So just you have to load "database" library from autoload.php file. So let's simply add as like i added this line on autoload.php file.
$autoload['libraries'] = array('database');
Now you can try and check did you solved or not.
I hope you found your best solution...