Declare foreign key very necessary for the referential integrity. As for a second question, Laravel official documentation recommended declare in the following form:

public function comments()
{
    return $this->hasMany('App\Comment');
}

Note, a correct form declare foreign key use migration:

$table->integer('user_id')->unsigned()->index();
$table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');

P.S sorry for my English :)

You should try this:

1)

$table->integer('user_id')->unsigned();
$table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');

2)

public function Comment()
{
      return $this->hasMany('App\Comment');
}

OR

use App\Comment;
 
public function Comment()
 {
       return $this->hasMany(Comment::clss);
 }