Store foreign key value in laravel

In Migration Table:

Schema::create('passws', function (Blueprint $table) {
        $table->increments('id');
        $table->integer('regist_id')->unsigned()->nullable();
        $table->foreign('regist_id')->references('id')->on('regists');
        $table->string('age');
        $table->timestamps();
    });
}

Regist Model - Mass Assignment Defined.

 public function pass(){
    return $this->hasOne('App\Passw');
}

In Controller

$name = $request->name;
$task = Regist::whereName($name)->get();
    foreach ($task as $tasks){
        $passw1->regist_id = $tasks->id;
    }
 $passw1->age = $request->age;
 $regist = new Regist();
 $regist->pass()->save($passw1);

When i store data, only age is getting stored but regist_id stored as null, (no error message or nothing). Here i'm sure controller shows "regist_id" => 1 and "age" => 25 when i use dd($passw1); regist_id is only not getting stores in DB table.

Where am i doing an error??

Try this

$name = $request->name;

$regist = Regist::whereName($name)->first(); //it will give you a Regist model

$regist->pass()->create([    
     'age' => $request->age
]);

If you have more than one Regist for $name then try this

$regists = Regist::whereName($name)->get(); //it will give you a collection of Regist model

foreach ($regists as $regist){
    $regist->pass()->create([    
          'age' => $request->age
    ]);
}

Check document here https://laravel.com/docs/5.6/eloquent-relationships#the-create-method