c# - Entity Framework 6 : Insert or Update

Question

我想使用 Entity Framework 在表中插入或更新一行，具体取决于表中是否已存在主键(字符串)。

我正在使用以下代码来执行我从此处获得的代码:Insert or update pattern

private void InsertOrUpdate(ServiceMonitoring sm)
{
    try
    {
        using (var context = new MyDBEntities())
        {
            context.Entry(sm).State = sm.serviceName == null ?
                EntityState.Added :
                EntityState.Modified;
            context.SaveChanges();
        }
        log.Info("ServiceMonitoring updated");
    }
    catch (Exception ex)
    {
        log.Error("Error updating ServiceMonitoring");
        log.Debug(ex.Message);
    }

}

当行(例如 serviceName = "MyService")已经存在时它工作正常。然后执行更新。但是，如果该行不存在，则 INSERT 失败并且我收到以下错误消息:

Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded.

非常感谢任何帮助!

Answer 1

更新或插入模式与 MSDN 示例中的主键一起工作的原因是因为它被标记为 Identity 字段。当您将实体添加到 DbContext 时，EF 会为 Id 字段提供默认值 0，并且数据库会在执行 INSERT 时为该行分配一个唯一值。

您已尝试错误地调整“更新或插入”模式以使用属性 serviceName，解决方案是使用您引用的 MSDN 文章提供的模式，或执行一个查询。

为了完整起见，为什么您的改编不起作用？ 因为您传入了一个 ServiceMonitoring 对象并期望 context.Entry(sm) 查询您的数据库以获取正确的数据行。它没有，先生。因此，在您的情况下，EntityState.Added 基本上是无法访问的代码。

private void InsertOrUpdate(ServiceMonitoring sm)  
{
    try
    {
        //this brand new DbContext instance has not queried your database, and is not tracking any objects!
        using (var context = new MyDBEntities())  
        {
            //if (sm.serviceName is null) <- should never be null, as it's the ServiceMonitoring object you are passing into the function.
            //The DbContext.Entry() does not know whether or not this actually exists in the database, it only allows you to inform
            //EF about the state of the object.
            context.Entry(sm).State = sm.serviceName == null ?  //<- Always false (unless you *really* want a serviceName to not have a name)
                EntityState.Added : // <- code unreachable
                EntityState.Modified; //This is where the exception occurs, your code is always generating an UPDATE statement.
                                        //When the entry exists in the database, EF's generated UPDATE statement succeeds.
                                        //When the entry does not exist, the UPDATE statement fails.
            context.SaveChanges();
        }
        log.Info("ServiceMonitoring updated");
    }
    catch (Exception ex)
    {
        log.Error("Error updating ServiceMonitoring");
        log.Debug(ex.Message);
    }
}