加载一个model主要做了以下几件事：

• 找寻并加载存放对应model类的文件
• 实例化model
• 把实例化的类挂在到controller实例上

public function model($model, $name = '', $db_conn = FALSE)
{
    if (empty($model))
    {
        return $this;
    }
    elseif (is_array($model))
    {
        foreach ($model as $key => $value)
        {
            is_int($key) ? $this->model($value, '', $db_conn) : $this->model($key, $value, $db_conn);
        }

        return $this;
    }

    // 有可能在子目录中
    $path = '';
    if (($last_slash = strrpos($model, '/')) !== FALSE)
    {
        $path = substr($model, 0, ++$last_slash);
        $model = substr($model, $last_slash);
    }

    // 默认在controller实例上的挂载名为model名
    if (empty($name))
    {
        $name = $model;
    }

    // 预防重名
    if (in_array($name, $this->_ci_models, TRUE))
    {
        return $this;
    }
    $CI =& get_instance();
    if (isset($CI->$name))
    {
        throw new RuntimeException('The model name you are loading is the name of a resource that is already being used: '.$name);
    }

    if ($db_conn !== FALSE && ! class_exists('CI_DB', FALSE))
    {
        if ($db_conn === TRUE)
        {
            $db_conn = '';
        }

        $this->database($db_conn, FALSE, TRUE);
    }

    // 加载基类，包括框架提供的```CI_Model```和自行扩展的model基类
    if ( ! class_exists('CI_Model', FALSE))
    {
        $app_path = APPPATH.'core'.DIRECTORY_SEPARATOR;
        if (file_exists($app_path.'Model.php'))
        {
            require_once($app_path.'Model.php');
            if ( ! class_exists('CI_Model', FALSE))
            {
                throw new RuntimeException($app_path."Model.php exists, but doesn't declare class CI_Model");
            }
        }
        elseif ( ! class_exists('CI_Model', FALSE))
        {
            require_once(BASEPATH.'core'.DIRECTORY_SEPARATOR.'Model.php');
        }

        // 加载自行扩展的model基类
        $class = config_item('subclass_prefix').'Model';
        if (file_exists($app_path.$class.'.php'))
        {
            require_once($app_path.$class.'.php');
            if ( ! class_exists($class, FALSE))
            {
                throw new RuntimeException($app_path.$class.".php exists, but doesn't declare class ".$class);
            }
        }
    }

    // 寻找并加载model所在文件
    $model = ucfirst($model);
    if ( ! class_exists($model, FALSE))
    {
        foreach ($this->_ci_model_paths as $mod_path)
        {
            if ( ! file_exists($mod_path.'models/'.$path.$model.'.php'))
            {
                continue;
            }

            require_once($mod_path.'models/'.$path.$model.'.php');
            if ( ! class_exists($model, FALSE))
            {
                throw new RuntimeException($mod_path."models/".$path.$model.".php exists, but doesn't declare class ".$model);
            }

            break;
        }
        // 可能文件存在但对应的类不存在
        if ( ! class_exists($model, FALSE))
        {
            throw new RuntimeException('Unable to locate the model you have specified: '.$model);
        }
    }
    // 要保证加载的类是```CI_Model```的子类
    elseif ( ! is_subclass_of($model, 'CI_Model'))
    {
        throw new RuntimeException("Class ".$model." already exists and doesn't extend CI_Model");
    }

    $this->_ci_models[] = $name;
    // 实例化model，并挂载到controller实例上
    $CI->$name = new $model();
    // 支持链式调用
    return $this;
}

实现核心功能的代码其实并不多，大多数代码都是在容错。

我个人觉得写的非常好的一段是：

if (empty($model))
{
    return $this;
}
elseif (is_array($model))
{
    foreach ($model as $key => $value)
    {
        is_int($key) ? $this->model($value, '', $db_conn) : $this->model($key, $value, $db_conn);
    }

    return $this;
}

这一段代码支持了array传参，支持了一次加载多个model。这种API设计方式其实非常常见，我在前端的相关实现也是用到了类似的思路。