I do not think that this is the best approach but it works for me.

$factory->define(App\UserSkill::class, function (Faker\Generator $faker) {    return [        'user_id' => factory(App\User::class)->create()->id,        'skill_id' => factory(App\Skill::class)->create()->id,    ];});

If you do not want to create a model just for the pivot table, you can insert it manually.

DB::table('user_skill')->insert(    [        'user_id' => factory(App\User::class)->create()->id,        'skill_id' => factory(App\Skill::class)->create()->id,    ]);

Or, with random existing values.

DB::table('user_skill')->insert(    [        'user_id' => User::select('id')->orderByRaw("RAND()")->first()->id,        'skill_id' => Skill::select('id')->orderByRaw("RAND()")->first()->id,    ]);

For those who are using laravel 8.x and are looking for a solution to a problem like this;

In laravel 8.x you can feed your pivot using Magic Methods,For example if you have a belongsToMany relation named "userSkills" in User model, you should feed the pivot table this way:

User::factory()->hasUserSkills(1, ['skills' => 'draw'])->create();

You can find the documentation here

I had a similar problem and I resolved this way on Laravel testing.

Don't need to create a new UserSkills Model:

Version Laravel 5.7

Database

users                user_skill                               skills+----------------+   +------------------------------------+   +-----------------+| id  | name     |   | user_id | section_id | state_skill |   | id  | skills    |+----------------+   +------------------------------------+   +-----------------+| 1   | Alex     |   |         |            |             |   | 1   | draw      ||----------------|   |----------------------|-------------|   |-----------------|| 2   | Lucy     |   |         |            |             |   | 2   | program   ||----------------|   |----------------------|-------------|   |-----------------|| 3   | Max      |   |         |            |             |   | 3   | social    ||----------------|   |----------------------|-------------|   +-----------------+| 4   | Sam      |   |         |            |             |+----------------+   +----------------------+-------------+

User.php

<?phpnamespace App;use Illuminate\Notifications\Notifiable;use Illuminate\Contracts\Auth\MustVerifyEmail;use Illuminate\Foundation\Auth\User as Authenticatable;use Illuminate\Database\Eloquent\SoftDeletes;class User extends Authenticatable{    use Notifiable;    use SoftDeletes;        public function skills()    {        return $this->belongsToMany('App\Skill')                ->withTimestamps()                ->withPivot('state_skill');    }}

DataBaseTest.php

<?phpnamespace Tests\Unit;use Tests\TestCase;use Illuminate\Foundation\Testing\WithFaker;use Illuminate\Foundation\Testing\RefreshDatabase;use Illuminate\Support\Facades\Schema;class DataBaseTest extends TestCase{    /**     * @test     */        public function test_create_user_skill()    {       // Create DataBase       $users = factory(\App\User::class, 1)           ->create()           ->each(function ($user) {                // Create Models Support                $skill = factory(\App\Skill::class)->create();                                // Create Pivot with Parameters                $user->skills()->attach($skill->id,[                    'state_skill' => 'ok'                ]);             });        // Testing        // ...        $this->assertTrue(true);    }}