Laravel Factory Conditonal statement

I currently have the following code in my posts factory and would like to fill the user_id field with the id of users with the role of writers in my users table.

$factory->define(Post::class, function (Faker $faker) {
   return [
       'title'      => $faker->sentences(1, true),
       'body'       => $faker->sentences(3, true),
       'user_id'    => function() use ($faker) {
           if (User::count())
               return $faker->randomElement(User::pluck('id')->toArray());
           else return factory(User::class)->create()->id;
       },
   ];
});

any help on how to implement this will be appreciated.