What is the difference between library and model i

问题:

I am new to CI. Before I had worked extensively in RoR and Symfony.

I am unable to understand why exactly CI provides Library and Models. Most of the CI code I examined generally used Library as a Wrapper around Model. Models are exclusively kept for talking to database.

Can someone throw light on this ?

回答1:

There are probably other schools of thought on this, but for me it's as follows:

Models

Models are tightly coupled to your application, making direct references to your DB schema/architecture/file paths etc.

Libraries

Libraries are loosely coupled. They should be treated like 3rd party add-ons and should make no assumptions about your app or your system. You should be able to "drop in" libraries at will with a minimum amount of configuration. Indeed, the opposite should be true, items in your library folder should be able to be dropped into any other CI application.

回答2:

The basci thought on Libraries was a way to extend Codeigniter's functionality through classes.

If you compare the blank anatomy of a Library and a Model in Codeigniter, you will see that Models extend the CI_MODEL which allows you to access Codeigniter's native resources (for Example $this->db).

Libraries do not offer that basic access and they also do not need to be extended through the CI_MODEL.

Models are made to depict and interact with data (mostly abstracted from databases such as mysql).

Take a look at the basic Library

<?php defined('BASEPATH') OR exit('No direct script access allowed');

class Library{

   public function MyFunction(){
      //do something...
   }

}

...and the Model anatomy

<?php defined('BASEPATH') OR exit('No direct script access allowed');

class User_model extends CI_Model {

   public function __construct(){
        parent::__construct();            
    }

    public function MyFunction(){
        //do something...
    }
}

In most cases the Models are prepared to use Ressources to access databases or other functionalities. Libraries don't. You need to include or extend the Ressources manually if you need them:

//Create an CI instance
$CI =& get_instance();

Libraries are tool collections and function extensions while Models are natively ideal for abstracting and interacting with data coming from databases.