Question

在MS SQL Server中，我有一个表，该表包含呼叫联系人的历史记录（这是另一个表）。通过EF访问，实体如下：

public partial class CallbackHistory
{
    public int HistoryId { get; set; }
    public int CompanyId { get; set; }
    public int CallerId { get; set; }
    public DateTime LastCallTimeStamp { get; set; }

    public virtual CompanyDiary Caller { get; set; }
    public virtual Company Company { get; set; }
}

和

public partial class CompanyDiary
{
    public CompanyDiary()
    {
        DatiCallbackHistory = new HashSet<DatiCallbackHistory>();
    }
    public int CallerId { get; set; }
    public string NickName { get; set; }
    public string PhoneNumber { get; set; }
    public string Email { get; set; }
    public int CompanyId { get; set; }

    public virtual Company Company { get; set; }
    public virtual ICollection<CallbackHistory> CallbackHistory { get; set; }
}

我需要获取按日期降序对个人号码最近5次通话的列表。

很遗憾，我想到了以下无法转换为SQL的查询：

var historyOfCalls = await
                    context.CallbackHistoryDbSet
                    .Include(historyEntry => historyEntry.Caller)
                    .Where(historyEntry => historyEntry.CompanyId == companyId)
                    .GroupBy(s => s.Caller.PhoneNumber)
                    .Select(s => s.OrderByDescending(historyEntry => historyEntry.LastCallTimeStamp).FirstOrDefault())
                    .Take(5)
                    .AsNoTracking()
                    .ToListAsync(cancellationToken).ConfigureAwait(false);

这是我得到的错误：

System.AggregateException
  HResult=0x80131500
  Message=One or more errors occurred. (The LINQ expression '(GroupByShaperExpression:
KeySelector: (c.PhoneNumber), 
ElementSelector:(EntityShaperExpression: 
    EntityType: CallbackHistory
    ValueBufferExpression: 
        (ProjectionBindingExpression: EmptyProjectionMember)
    IsNullable: False
)
)
    .OrderByDescending(historyEntry => historyEntry.LastCallTimeStamp)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.)
  Source=System.Private.CoreLib

Inner Exception 1:
InvalidOperationException: The LINQ expression '(GroupByShaperExpression:
KeySelector: (c.PhoneNumber), 
ElementSelector:(EntityShaperExpression: 
    EntityType: CallbackHistory
    ValueBufferExpression: 
        (ProjectionBindingExpression: EmptyProjectionMember)
    IsNullable: False
)
)
    .OrderByDescending(historyEntry => historyEntry.LastCallTimeStamp)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

问题似乎出在我对导航属性进行分组的事实上。

我可以重写此查询以使其可转换为SQL吗？

我不知道何时使用此查询切换到Linq to objects，因为我已经有一个呼叫ToListAsync的电话。我尝试将其移至查询中Select之后，但无法编译

Answer 1

在查询中更早地调用ToListAsync将导致所有其他linq语句无法编译，因为ToListAsync将返回一个Task，因此本质上您将需要首先等待结果或调用.Result（这将阻塞当前线程）。我的建议是将查询拆分为：

获取数据
投影数据

例如

    var historyOfCalls = await context.CallbackHistoryDbSet
        .Include(historyEntry => historyEntry.Caller)
        .Where(historyEntry => historyEntry.CompanyId == companyId)
        .AsNoTracking()
        .ToListAsync(cancellationToken).ConfigureAwait(false);

    var projection = historyOfCalls 
        .GroupBy(s => s.Caller.PhoneNumber);

请记住，通过呼叫分组，您会得到分组，因此在呼叫Select时，您具有Key属性（电话号码）和and value属性。我建议您通过使用调用方DbSet来反转查询，并包括其调用方历史记录，然后从那里进行分组，并使用分组依据上的一些重载来选择将值更正为TV。

    var callers = await context.CompanyDiaryDbSet
        .Include(c => c.CallbackHistory)
        .Where(c=> c.CompanyId == companyId)
        .AsNoTracking()
        .ToListAsync(cancellationToken).ConfigureAwait(false);

EF 3.1：克服LINQ GroupBy SQL转换问题

1 个答案: