如何在 EF Core 3.1 中使用 Linq 表达式加入 GroupBy

Question

这是我的 SQL 查询，我在 linqpad 中对其进行了测试，它有效，但在 EF Core 3.1 中无效：

  from v in JournalVoucherLines
  group v by v.AccountId into vg
  join bp in Parties on vg.FirstOrDefault().AccountId equals bp.AccountId
  select new
  {
    name = bp.FullName,
    key = vg.Key,
    Creditor = vg.Sum(v => v.Credit),
    Deptor = vg.Sum(v => v.Debit),
    RemainAmount = vg.Sum(v => v.Credit) - vg.Sum(v => v.Debit)
  }

当我在 EF Core 中使用查询时，出现此异常：

The LINQ expression '(GroupByShaperExpression: KeySelector: (j.AccountId), ElementSelector:(EntityShaperExpression: EntityType: JournalVoucherLine ValueBufferExpression: (ProjectionBindingExpression: EmptyProjectionMember) IsNullable: False ) ).FirstOrDefault()'
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. "

此查询的最佳做法是什么？

Answer 1

首先，不要在 GroupBy 结果上使用 FirstOrDefault() 这样的方法 - 它们不可翻译。仅使用键和聚合函数（因为这是 SQL GROUP BY 运算符支持的）。

其次，对包含所需 key/aggregates 的 GroupBy 结果使用临时投影 (Select)，然后将其加入另一个实体 (表）以获取最终投影所需的附加信息。

例如

from v in JournalVoucherLines
group v by v.AccountId into vg
select new // <-- temporary projection with group by fields needed
{
    AccountId = vg.Key,
    Credit = vg.Sum(v => v.Credit),
    Debit = vg.Sum(v => v.Debit),
} into vg
join bp in Parties on vg.AccountId equals bp.AccountId // <-- additional join(s)
select new
{
    name = bp.FullName,
    key = vg.AccountId,
    Creditor = vg.Credit,
    Deptor = vg.Debit,
    RemainAmount = vg.Credit - vg.Debit
};

成功转换为

SELECT [p].[FullName] AS [name], [t].[AccountId] AS [key], [t].[c] AS [Creditor], [t].[c0] AS [Deptor], [t].[c] - [t].[c0] AS [RemainAmount]
FROM (
    SELECT [j].[AccountId], SUM([j].[Credit]) AS [c], SUM([j].[Debit]) AS [c0]
    FROM [JournalVoucherLines] AS [j]
    GROUP BY [j].[AccountId]
) AS [t]
INNER JOIN [Parties] AS [p] ON [t].[AccountId] = [p].[AccountId]

更新： 使用方法语法的相同 LINQ 查询甚至更简单：

var query = JournalVoucherLines
    .GroupBy(v => v.AccountId)
    .Select(vg => new
    {
        AccountId = vg.Key,
        Credit = vg.Sum(v => v.Credit),
        Debit = vg.Sum(v => v.Debit),
    })
    .Join(Parties, vg => vg.AccountId, bp => bp.AccountId, (vg, bp) => new
    {
        name = bp.FullName,
        key = vg.AccountId,
        Creditor = vg.Credit,
        Deptor = vg.Debit,
        RemainAmount = vg.Credit - vg.Debit
    });

但如果您需要更多连接，最好使用查询语法。

如何在 EF Core 3.1 中使用 Linq 表达式加入 GroupBy

How can I use Linq expression for Join with GroupBy in EF Core 3.1

linq

eager-loading

entity-framework-core

ef-core-3.1