EF 3.1：克服 LINQ GroupBy SQL 翻译问题 [重复]

Question

在 MS SQL Server 中，我有一个表，其中包含呼叫联系人的历史记录（即另一个表）。 EF访问的实体如下：

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; }
}

我需要按日期降序获取最近 5 次呼叫单个号码的列表。

很遗憾，我想出了以下无法转换为 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.

问题似乎在于我正在对导航属性进行分组。

我可以重写此查询以使其可转换为 SQL 吗？

我不知道什么时候用这个查询切换到Linq to objects 因为我已经接到了ToListAsync 的电话。我试图在查询中的Select 之后移动它，但它没有编译

Answer 1

在查询的前面调用 ToListAsync 将导致所有其他 linq 语句无法编译，因为 ToListAsync 将返回一个 Task 所以基本上你需要先等待结果或调用 .Result （这将是当前线程的阻塞） .我的建议是将查询拆分为：

1. 获取数据
2. 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);

记住，通过调用 group by 你会得到一个 Grouping，所以当调用 Select 时你有一个 Key 属性（电话号码）和一个 value 属性。我建议通过使用调用方 DbSet 并包含其调用方历史记录来反转您的查询，然后从那里分组并使用 group by 上的一些重载来选择将值更正到 TV 中。

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