问为IdentityUser添加外键，创建用户时提示主表外键冲突？

[NotMapped]
public class AppUser:IdentityUser
{
    [Required]
    public string RealName { get; set; } = default!;

    [Required]
    [ForeignKey("DepartmentId")]
    public Department Department { get; set; } = default!;
}

    public class Department
    {
        [Key]
        public int DepartmentId { get; set; }

        [Required]
        public string Name { get; set; } = default!;

        public Department? Parent { get; set; }
    }

if (ModelState.IsValid)
{
    var user = new AppUser { UserName = model.UserName, RealName = model.RealName};
    var dep = _departmentRepository.Departments.FirstOrDefault(d => d.DepartmentId == Id);
    user.Department = dep;
    
    if (model.Password != null)
    {
        var result = await userManager.CreateAsync(user, model.Password);
    }
    return View(model);
 }