Question

我有一个嵌套的数据模型，我想从中获取按顶级属性分组的聚合数据。我的模型例如：

public class Scan {
    public long Id {get; set;}
    public int ProjectId { get; set; }
    public int ScanUserId { get; set; }
    public ICollection<ScanItem>? Items { get; set; }
}

public class ScanItem
{
    public long Id { get; set; }
    public long InventoryScanId { get; set; }
    public double Qty { get; set; }
}

我想获取按Scan.ScanUserId分组的所有扫描，然后获取每个用户的ScanItems.Qty之和。我的查询看起来像这样，EF给出以下错误：

LINQ表达式'AsQueryable（（未处理参数：x）.Items）”通过“ NavigationExpandingExpressionVisitor”失败

from scan in Scans
        .Include(x=>x.ScanUser)
        .Include(x=>x.Items)
    group scan by new { scan.ScanUser.Name, scan.ScanUser.Id } into g
    select new
    {
        UserId = g.Key.Id,
        Name = g.Key.Name,
        LastSyncTime = g.Max(x => x.ScanDate),
        ScanItems = g.Sum(x=>x.Items.Sum(i=>i.Qty))
    }

如何在不对客户端进行评估的情况下在嵌套表的属性上运行聚合函数？

Answer 1

EF Core仍然无法转换GroupBy结果（分组）上的嵌套聚合。

您必须通过使用GroupBy（或查询语法element中的group element by key）的元素选择器来预先计算嵌套的聚合：

from scan in Scans
group new { scan.ScanDate, Qty = scan.Items.Sum(i => i.Qty) } // <--
by new { scan.ScanUser.Name, scan.ScanUser.Id } into g
select new
{
    UserId = g.Key.Id,
    Name = g.Key.Name,
    LastSyncTime = g.Max(x => x.ScanDate),
    ScanItems = g.Sum(x => x.Qty) // <--
}

**更新：对于SqlServer，**上述LINQ查询转换为SQL查询，如下所示：

 SELECT [s1].[Id] AS [UserId], [s1].[Name], MAX([s0].[ScanDate]) AS [LastSyncTime], SUM((
      SELECT SUM([s].[Qty])
      FROM [ScanItem] AS [s]
      WHERE [s0].[Id] = [s].[InventoryScanId])) AS [ScanItems]
  FROM [Scan] AS [s0]
  INNER JOIN [ScanUser] AS [s1] ON [s0].[ScanUserId] = [s1].[Id]
  GROUP BY [s1].[Name], [s1].[Id]

如注释中所述，它生成SQL执行异常“无法对包含聚合或子查询的表达式执行聚合函数。” 。

因此，您真的需要另一种方法-在分组之前使用left join展平结果集，然后对该结果集进行分组/汇总：

from scan in Scans
from item in scan.Items.DefaultIfEmpty() // <-- left outer join
group new { scan, item } by new { scan.ScanUser.Name, scan.ScanUser.Id } into g
select new
{
    UserId = g.Key.Id,
    Name = g.Key.Name,
    LastSyncTime = g.Max(x => x.scan.ScanDate),
    ScanItems = g.Sum(x => (double?)x.item.Qty) ?? 0
};

现在可以转换为希望有效的SqlServer SQL查询：

  SELECT [s1].[Id] AS [UserId], [s1].[Name], MAX([s].[ScanDate]) AS [LastSyncTime], COALESCE(SUM([s0].[Qty]), 0.0E0) AS [ScanItems]
  FROM [Scan] AS [s]
  LEFT JOIN [ScanItem] AS [s0] ON [s].[Id] = [s0].[InventoryScanId]
  INNER JOIN [ScanUser] AS [s1] ON [s].[ScanUserId] = [s1].[Id]
  GROUP BY [s1].[Name], [s1].[Id]

ef核心嵌套表聚合函数给出“ NavigationExpandingExpressionVisitor失败”

1 个答案: