EntityFramework Core - 复制实体并将其放回数据库

Question

是否有最佳实践来复制实体,根据用户输入对其进行一些更改,然后将其重新插入数据库？

其他一些Stackoverflow线程已经提到过,即使数据库中存在相同的主键,EF也会为您处理插入新对象,但我不太确定EF Core是如何处理它的.每当我尝试复制一个对象时,我都会收到错误

Cannot insert explicit value for identity column in table when IDENTITY_INSERT is set to OFF

基本上我只需要一种简洁的方法来复制对象,根据用户输入对其进行一些更改,然后将该副本插回到数据库中,并使Id自动递增.有没有最佳实践或简单的方法,而无需手动将属性设置为null或空？

编辑:从数据库中检索对象的示例代码:

    public Incident GetIncidentByIdForCloning(int id)
    {
        try
        {
            return _context.Incident.Single(i => i.IncidentId == id);
        }
        catch
        {
            return null;
        }
    }

检索对象后的代码(因为某些字段是自动生成的,如RowVersion,它是一个时间戳):

public IActionResult Clone([FromBody]Incident Incident)
    {
        var incidentToCopy = _incidentService.IncidentRepository.GetIncidentByIdForCloning(Incident.IncidentId);
        incidentToCopy.IncidentTrackingRefId = _incidentService.IncidentRepository.GetNextIdForIncidentCategoryAndType(
            Incident.IncidentCategoryLookupTableId, Incident.IncidentTypeLookupTableId).GetValueOrDefault(0);
        incidentToCopy.RowVersion = null;
        incidentToCopy.IncidentId = 0; //This will fail with or without this line, this was more of a test to see if manually setting would default the insert operation, such as creating a brand new object would normally do.
        incidentToCopy.IncidentCategoryLookupTableId = Incident.IncidentCategoryLookupTableId;
        incidentToCopy.IncidentTypeLookupTableId = Incident.IncidentTypeLookupTableId;
        var newIncident = _incidentService.IncidentRepository.CreateIncident(incidentToCopy);
...

我意识到我可以制作一个全新的对象并进行左手复制,但这看起来非常低效,我想知道EF Core是否提供了更好的解决方案.

Answer 1

因此,我在创建这个之前偶然发现了"可能重复"的线程,比我最初偶然发现的那样,并且我忽略了一个不那么高度推崇的解决方案,基本上只是抓住了所有的值.一次从数据库中检索对象时 - 它不会在进程中检索对该对象的引用.我的代码现在看起来像这样:

try
{
    var incidentToCopy = _context.Incident.Single(i => i.IncidentId == id);
    return (Incident) _context.Entry(incidentToCopy).CurrentValues.ToObject();
}

Answer 2

在您的IncidentRepository课程中，尝试Incident通过 using获取 ，AsNoTracking并且在添加时应将其作为新实体进行跟踪。

public void Clone(int id)
{
    // Prevent tracking changes to the object.
    var incident = _context.AsNoTracking().SingleOrDefault(i => i.Id == id);

    // Setting back to 0 should treat the object Id as unset.
    incident.Id = 0;

    // Add the Incident while it is untracked will treat it as a new entity.
    _context.Incidents.Add(incident);
    _context.SaveChanges();
}