artisan db:seed fails with Class xxx does not exist

Did you try

composer dump-autoload

or try this

<?php

use Illuminate\Database\Seeder;
use App\NiceAction;
use Illuminate\Database\Eloquent\Model; // <- added this

class NiceActionSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        $nice_action = new NiceAction();
        $nice_action->name = "Greet";
        $nice_action->niceness = 3;
        $nice_action->save();

        $nice_action = new NiceAction();
        $nice_action->name = "Hug";
        $nice_action->niceness = 6;
        $nice_action->save();

        $nice_action = new NiceAction();
        $nice_action->name = "Kiss";
        $nice_action->niceness = 12;
        $nice_action->save();       
    }
}

<?php

use Illuminate\Database\Seeder;
use Illuminate\Database\Eloquent\Model; // <- added this


class DatabaseSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        $this->call(NiceActionSeeder::class);
    }
}

dump-autoload solved the problem. thank you. But, could you explain why I need to do this before db:seed ?

There is article about that

http://developed.be/2014/08/29/composer-dump-autoload-laravel/

http://stackoverflow.com/a/33974248

in short ... it regenerates list of all files and classes which must be included in you application