更新现有记录时出现 Entity Framework Core 错误

Question

在我的ASP.NET-Core Code First project 中，我在以下Action Method 中的SaveChangesAsync() 上收到以下错误：

错误

DbUpdateConcurrencyException: Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded

动作方法：

public async Task<IActionResult> UpdateWeekDay(int iWeekDay)
{
       if (ModelState.IsValid)
       {
            WeekModel oWeekModel = new WeekModel();
            oWeekModel.DayOfWeek= iWeekDay;
            _context.Update(oWeekModel);
            await _context.SaveChangesAsync();
            return View();
        }
}

型号：

public class WeekModel
{
    [Key]
    public int WeekId { get; set; }
    public int DayOfWeek { get; set; }
}

注意：SQL Server 2014 Db中对应的表Weeks有WeekId as an identity column and as the PK。而且，该表只包含一条记录。

更新：

按照这个Post from user sstan，我在上面的Action Method中尝试了以下。它不会给我一个错误，但也不会更新数据库：

WeekModel oWeekModel = new WeekModel();
_context.WeekModel.Attach(oWeekModel);
oWeekModel.DayOfWeek= iWeekDay;
await _context.SaveChangesAsync();

Answer 1

根据@Tseng 和@ademcaglin 和this post from @sstan 的建议，我能够解决以下问题。 注意：感谢上述用户（感谢这些用户）：

public async Task<IActionResult> UpdateWeekDay(int iWeekDay)
{
   if (ModelState.IsValid)
   {
      WeekModel oWeekModel = _context.WeekModel.Where(s => s.DayOfWeek > 0).FirstOrDefault<CurrentYear>();
      _context.WeekModel.Attach(oWeekModel);
      oWeekModel.DayOfWeek= iWeekDay;
      await _context.SaveChangesAsync();
      return View();
   }
}