Integrity constraint violation: 1452 Cannot add
  or update a child row: a foreign key constraint fails (`twitter`.
  `tweets`, CONSTRAINT `tweets_user_id_foreign` FOREIGN KEY (`user_
  id`) REFERENCES `users` (`id`) ON DELETE CASCADE)

<?php

use IlluminateSupportFacadesSchema;
use IlluminateDatabaseSchemaBlueprint;
use IlluminateDatabaseMigrationsMigration;

class CreateTweetsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('tweets', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('user_id')->unsigned()->index();
            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
            $table->string('body', 140);
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('tweets');
    }
}

<?php

namespace App;

use IlluminateDatabaseEloquentModel;

class Tweet extends Model
{
    protected $fillable = [
        'user_id', 'body',
    ];
}

<?php

use AppTweet;
use IlluminateDatabaseSeeder;

class TweetsTableSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        factory(Tweet::class, 10)->create([
        'user_id' => 2
        ]);
    }
}

<?php

/*
|--------------------------------------------------------------------------
| Model Factories
|--------------------------------------------------------------------------
|
| Here you may define all of your model factories. Model factories give
| you a convenient way to create models for testing and seeding your
| database. Just tell the factory how a default model should look.
|
*/

/** @var IlluminateDatabaseEloquentFactory $factory */
$factory->define(AppUser::class, function (FakerGenerator $faker) {
    static $password;

    return [
        'name' => $faker->name,
        'email' => $faker->unique()->safeEmail,
        'password' => $password ?: $password = bcrypt('secret'),
        'remember_token' => str_random(10),
    ];
});

$factory->define(AppTweet::class, function (FakerGenerator $faker) {
    return [
        'body' => $faker->realText(140),
    ];
});