我正在.net Core 3.0中处理Entity Framework Core 3.0 但是当我尝试更新此项目（仅此类，在db first模型中）时，它给了我这个错误

Sql异常。无法更新标识列“ IDNRR”

我只是从X表中获取一个项目，然后使用context.Update命令保存它

单元测试异常

var c = _dbContext.TblWoTaskExtServices.SingleOrDefault(x => x.Idnrr == 15273778);
_dbContext.Update(c);
_dbContext.SaveChanges(); -> execute this give me the sql exception

类定义

modelBuilder.Entity<TblWoTaskExtServices>(entity =>
{
    entity.HasKey(e => new { e.ExternalServiceCode,e.IdWotask });

    entity.ToTable("tbl_WO_TASK_EXT_SERVICES");

    entity.HasIndex(e => e.Idnrr)
        .Hasname("IX_tbl_WO_TASK_EXT_SERVICES")
        .IsUnique();

    entity.Property(e => e.ExternalServiceCode)
        .HasMaxLength(255)
        .IsUnicode(false);

    entity.Property(e => e.IdWotask).HasColumnName("IdWOTask");

    entity.Property(e => e.DateIns).HasColumnType("smalldatetime");

    entity.Property(e => e.DateMod).HasColumnType("smalldatetime");

    entity.Property(e => e.ExternalServiceDescription).HasColumnType("text");

    entity.Property(e => e.Idnrr)
        .HasColumnName("IDNRR")
        .ValueGeneratedOnAdd();

    entity.Property(e => e.Notes).HasColumnType("text");

    entity.Property(e => e.TotalCost).HasColumnType("decimal(18,2)");

    entity.Property(e => e.TotalCostFinal).HasColumnType("decimal(18,2)");

    entity.HasOne(d => d.IdCompanyNavigation)
        .WithMany(p => p.TblWoTaskExtServices)
        .HasPrincipalKey(p => p.Idnrr)
        .HasForeignKey(d => d.IdCompany)
        .OnDelete(DeleteBehavior.ClientSetNull)
        .HasConstraintName("FK_tbl_WO_TASK_EXT_SERVICES_tbl_COMPANIES");

    entity.HasOne(d => d.IdCostClassnavigation)
        .WithMany(p => p.TblWoTaskExtServices)
        .HasPrincipalKey(p => p.Idnrr)
        .HasForeignKey(d => d.IdCostClass)
        .HasConstraintName("FK_tbl_WO_TASK_EXT_SERVICES_tbl_COST_CLASSES");

    entity.HasOne(d => d.IdCurrencyNavigation)
        .WithMany(p => p.TblWoTaskExtServices)
        .HasPrincipalKey(p => p.Idnrr)
        .HasForeignKey(d => d.IdCurrency)
        .HasConstraintName("FK_tbl_WO_TASK_EXT_SERVICES_tbl_Currencies");
    });
}