更正EF 6中AddOrUpdate()的多个条件

错误应在EF 6.2.0中修复: https://github.com/aspnet/EntityFramework6/issues/9

解决方法:安装 EF beta version

ReceivedMessage record = db.ReceivedMessages.FirstOrDefault(p => p.PatientId == status.PatientId &&
                                                                    p.DialogId == status.ConversationId);
//  Add new if not exists
bool newRecord = false;
if (record == null)
{
    record = new ReceivedMessage();
    record.Id = Guid.NewGuid();
    newRecord = true;
}

// Save fields
// ...

// Save to DB
if (newRecord)  // Workaround for EF 6.1 bug: https://stackoverflow.com/a/44811951/630169
    db.ReceivedMessages.Add(record);
else
    db.Entry(record).CurrentValues.SetValues(record);

db.SaveChanges();