Mageno2 Model的创建不同于其他框架，需要3个不同目录层级的文件

例如需要为表hello_test创建model：

1、app/code/Hello/Test/Model/Test.php

<?php

namespace Hello\Test\Model;

class Test extends \Magento\Framework\Model\AbstractModel
{
    protected $_idFieldName = 'id';

    protected function _construct()
    {
        $this->_init('\Hello\Test\Model\ResourceModel\Test');
    }
}

2、app/code/Hello/Test/Model/ResourceModel/Test.php

<?php

namespace Hello\Test\Model\ResourceModel;

class Test extends \Magento\Framework\Model\ResourceModel\Db\AbstractDb
{
    public function _construct()
    {
        // TODO: Implement _construct() method.
        $this->_init('hello_test', 'id');
    }
}

3、app/code/Hello/Test/Model/ResourceModel/Test/Collection.php

<?php

namespace Hello\Test\Model\ResourceModel\Test;

use \Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection;

class Collection extends AbstractCollection
{
    public function _construct()
    {
        $this->_init('\Hello\Test\Model\Test', '\Hello\Test\Model\ResourceModel\Test');
    }
}