A simple concept for basic JOIN/relationship support in MY_Model

customer.php

<?php

class Customer extends MY_Model {
    protected $relations = array(
        'addresses' => array(
            'join_on' => 'addressable_id',
            'where' => array('addresses.addressable_type' => 'customers')
        )
    );
}

// This...
$this->db->select('customers.*, addresses.*')->join('addresses', 'addresses.addressable_id = customers.id')->where('addresses.addressable_type', 'customers')->get();

// ...becomes this.
$this->customer->with_relation('addresses')->get_all();

user.php

<?php

class User extends MY_Model {
    protected $relations = array(
        'books' => 'id'
    );
}

// This...
$this->db->select('users.*, books.*')->join('books', 'books.id = users.id')->get();

// ...becomes this.
$this->user->with_relation('books')->get_all();