我的hasMany in laravel响应为空，我的模型是正确的

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    public function posts()
    {
        return $this->hasMany(Post::class);
    }
}

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Post extends Model
{
    protected $fillable = ['user_id', 'title', 'content'];
}

use App\Models\User;

public function getUserPosts($userId)
{
    $user = User::find($userId);
    if ($user) {
        $posts = $user->posts;
        return response()->json($posts);
    } else {
        return response()->json(['error' => 'User not found'], 404);
    }
}