Laravel 5 单元测试 - 在 null 上调用成员函数 connection()

This is my UserTest.php

<?php

namespace TestsUnit;

use TestsTestCase;
use IlluminateFoundationTestingDatabaseMigrations;
use IlluminateFoundationTestingDatabaseTransactions;

use AppUser;
use AppShop;

class UserTest extends TestCase
{

    protected $user, $shop;

    function __construct()
    {
        $this->setUp();
    }
    
    function setUp()
    {
        $user = new User([
            'id' => 1,
            'first_name' => 'John',
            'last_name' => 'Doe',
            'email' => 'JohnDoe@example.com',
            'password' => 'secret',
            'facebook_id' => null,
            'type' => 'customer',
            'confirmation_code' => null,
            'newsletter_subscription' => 0,
            'last_online' => null,
            'telephone' => null,
            'mobile' => null,
            'social_security_id' => null,
            'address_1' => null,
            'address_2' => null,
            'city' => null,
            'zip_code' => null,
            'signed_agreement' => 0,
            'is_email_confirmed' => 0
        ]);

        $user = User::find(1);
        $shop = new Shop([
            'id' => 1,
            'user_id' => $user->id,
            'name' => 'PureFoods Hotdog2',
            'description' => 'Something that describes this shop',
            'url' => null,
            'currency' => 'USD'
        ]);
        $user->shops()->save($shop);

        $shop = new Shop([
            'id' => 2,
            'user_id' => $user->id,
            'name' => 'PureFoods Hotdog',
            'description' => 'Something that describes this shop',
            'url' => null,
            'currency' => 'USD'
        ]);

        $user->shops()->save($shop);

        $this->user = $user;

    }

    /** @test */
    public function a_user_has_an_id(){
        $user =  User::find(1);
        $this->assertEquals(1, $user->id);
    }

    /** @test */
    public function a_user_has_a_first_name()
    {
        $this->assertEquals("John", $this->user->first_name);
    }

    /** @test */
    public function a_user_can_own_multiple_shops()
    {
        $shops = User::find(1)->shops()->get();
        var_dump($this->shops);
        $this->assertCount(2, $shops);
    }
}

看来，这个错误是由这行代码引起的:$user->shops()->save($shop); - 此代码在我的示例路由或控制器中运行时实际上有效，但在运行时抛出 errors在 phpunit

<?php

namespace App;

use IlluminateNotificationsNotifiable;
use IlluminateFoundationAuthUser as Authenticatable;

class User extends Authenticatable
{
    use Notifiable;

    protected $guarded = [ 'id' ];
    protected $table = "users";   

    /**
     * The attributes that should be hidden for arrays.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];


    /**
     * returns the Shops that this user owned
     */
    public function shops()
    {
        return $this->hasMany('AppShop');
    }
}

<?php

namespace App;

use IlluminateDatabaseEloquentModel;

class Shop extends Model
{
    protected $guarded = [ 'id' ];
    protected $table = "shops";

    /**
     * returns the Owner of this Shop
     */
    public function owner()
    {
        return $this->belongsTo('AppUser');
    }
}