1452 Cannot add or update a child row: a foreign key constraint fails

I'm trying to associate quests with levels for my game. Each level contains one or many quests. Each quest is associated to one level.

When running my DB seeds, I get

1452 Cannot add or update a child row: a foreign key constraint fails (clg_local
.quests, CONSTRAINT quests_level_id_foreign FOREIGN KEY (level_id) REFERENCES levels (id) ON DELETE CASCADE)

Migrations: this is one of my migrations for quests:

    Schema::create('quests', function(Blueprint $table)
    {
        $table->increments('id');
        $table->string('quest_name')->nullable();
        $table->unsignedInteger('level_id')->nullable();

Then when all of my tables are created, I run this migration to specify my FK relationships:

    Schema::table('quests', function(Blueprint $table)
    {
        $table->foreign('level_id')->references('id')->on('levels')->onDelete('cascade');
    });

Where it's failing in the seeding: QuestsSeeder

foreach (
            [
                [
                    'quest_name' => 'Quest 1',
                    'level_id' => 1,
                    'reward_xp' => 200,
                    'reward_items' => null,
                    'reward_money' => 200,
                    ...

] as $input
        ) {
            $quest      = Quest::create($input);
            $quests[]   = $quest;
        }

        return $quests;

If I comment out $table->foreign('level_id')->references('id')->on('levels')->onDelete('cascade');, it works fine, but then can't retrieve the quests per level as such:

$quests = Level::findOrFail(1)->quests();

I assume this is because I'm not allowed to add level_id => 1 when seeding... so I commented that out and it works:

            [
                'quest_name' => 'Quest 1',
                //'level_id' => 1,
                'reward_xp' => 200,
                'reward_items' => null,
                'reward_money' => 200,
            ],

But then how can I associate that quest entry with a level? I need to add the level id to the quest entry.

via Chebli Mohamed