Magento 2 Training Overriding a core class

Question

I'm trying to implement a basic m2 class override following the M2 Fundamentals Course and it is not working. Here is what I have in my Training/Test/etc/di.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">

<preference for="Magento\Catalog\Model\Product" type="Training\Test\Model\Testproduct" />

</config>

And inside the Training\Test\Model\Testproduct.php

<?php
namespace Training\Test\Model;

class Testproduct extends \Magento\Catalog\Model\Product
{

 public function getPrice() {
    return 3;
 }
} 

I understand it's better to do plugins / events, but my question is why is the above not working?

The module is installed and I also ran the following:

php bin/magento cache:flush
rm -Rf var/cache/*
rm -Rf var/di/
rm -Rf var/generation/*
rm -Rf var/page_cache/*
php bin/magento setup:di:compile

It also shows when runnning: php bin/magento module:status

List of enabled modules:
...
Training_Test

Answer 1

You can try this simple module to override \Magento\Catalog\Model\Product class. It 100% works on my local Magento 2 test environment.

Base folder: app\code

Directory tree:

└── Example
    └── OverrideModel
        ├── composer.json
        ├── etc
        │   ├── frontend
        │   │   └── di.xml
        │   └── module.xml
        ├── Model
        │   └── Product.php
        └── registration.php

File: Example/OverrideModel/composer.json

{
    "name": "example/overridemodel",
    "description": "OverrideModel module for Magento 2",
    "type": "magento2-module",
    "version": "1.0.0",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "require": {
        "php": "~5.5.0|~5.6.0|~7.0.0",
        "magento/module-catalog": "~100.0"
    },
    "autoload": {
        "files": [
            "registration.php"
        ],
        "psr-4": {
            "Example\\OverrideModel\\": ""
        }
    }
}

File: Example/OverrideModel/registration.php

<?php

use \Magento\Framework\Component\ComponentRegistrar;

ComponentRegistrar::register(ComponentRegistrar::MODULE, 'Example_OverrideModel', __DIR__);

File: Example/OverrideModel/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Example_OverrideModel" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Catalog"/>
        </sequence>
    </module>
</config>

File: Example/OverrideModel/etc/frontend/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Catalog\Model\Product" type="Example\OverrideModel\Model\Product" />
</config>

File: Example/OverrideModel/Model/Product.php

<?php

namespace Example\OverrideModel\Model;

class Product extends \Magento\Catalog\Model\Product {

    public function getPrice()
    {
        return 3;
    }
}