Laravel 9 Eloquent relations with UUID - php

Question

why the user_id is set to ? in Eloquent hasOne relation's sql query
"select * from `files` where `files`.`user_id` = ? and `files`.`user_id` is not null",
here's the model code
<?php
namespace App\Models;
use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Database\Eloquent\Concerns\HasUuids;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\SoftDeletes;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Laravel\Passport\HasApiTokens;
class User extends Authenticatable implements MustVerifyEmail
{
use HasApiTokens, HasFactory, SoftDeletes,HasUuids;
/**
* The attributes that are mass assignable.
*
* #var array<int, string>
*/
protected $fillable = [
'name',
'email',
];
protected $keyType = 'string';
public $incrementing = false;
/**
* The attributes that should be hidden for serialization.
*
* #var array<int, string>
*/
protected $hidden = [
'password',
'remember_token',
];
/**
* The attributes that should be cast.
*
* #var array<string, string>
*/
protected $casts = [
'email_verified_at' => 'datetime',
'id'=>'string',
];
public function profile_picture(){
return $this->hasOne(File::class);
}
}
and the file model
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Concerns\HasUuids;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
class File extends Model
{
use HasFactory,HasUuids;
protected $fillable = [
'name',
'content',
'extension',
];
protected $keyType = 'string';
public $incrementing = false;
public function user(){
return $this->belongsTo(User::class);
}
}
migrations:
Schema::create('users', function (Blueprint $table) {
$table->uuid('id')->primary()->unique();
$table->string('name');
$table->string('email');
$table->softDeletes($column = 'deleted_at', $precision = 0);
$table->rememberToken();
$table->timestamps();
});
Schema::create('files', function (Blueprint $table) {
$table->uuid('id')->primary()->unique();
$table->string('name');
$table->string('extension');
$table->uuid('user_id')->nullable();
$table->foreign('user_id')->references('id')->on('users');
$table->timestamps();
});
Instead of using auto-incrementing integers as your Eloquent model's primary keys, you may choose to use UUIDs instead. UUIDs are universally unique alpha-numeric identifiers that are 36 characters long.
code php explain the problem and the corect code.