无法翻译 EF Core 3 Linq

Question

我尝试在 ef core 3.0 中构建一个从数据库服务器获取完整进程的查询

 IEnumerable<int> stIds = stateIds;
  var rtables = await db.Order.
                Join(db.OrderDetail, order => order.OrderId, orderdetail => orderdetail.OrderId, (order, orderdetail) => new { order, orderdetail }).
                Where(x => x.order.SellerId == sellerId && stIds.Contains(x.orderdetail.OrderStateId)&&x.order.RtableId != null)
                .GroupBy(x =>             
                x.order.RtableId               
                )
                .Select(x => new RtableState { RtableId = x.Key ?? 0, OrderStateId = x.OrderByDescending(x => x.orderdetail.OrderStateId).Select(x => x.orderdetail.OrderStateId).FirstOrDefault() }).ToListAsync();

我收到这个错误：

{ "Message": "Processing of the LINQ expression 'AsQueryable<<>f__AnonymousType52>(OrderByDescending<<>f__AnonymousType52, int>(\r\n source: NavigationTreeExpression\r\n Value: default(IGrouping, <>f__AnonymousType52>)\r\n Expression: (Unhandled parameter: e), \r\n keySelector: (x) => x.orderdetail.OrderStateId))' by 'NavigationExpandingExpressionVisitor' failed. This may indicate either a bug or a limitation in EF Core. See https://go.microsoft.com/fwlink/?linkid=2101433 for more detailed information.", "Inner"：“” }

我知道查询对于 EF Core 3.0 来说太复杂了，但这是一个错误还是应该不起作用？

我的解决方案是拆分请求。

IEnumerable<int> stIds = stateIds;
            var rtableStatesServer = await db.Order.
                Join(db.OrderDetail, order => order.OrderId, orderdetail => orderdetail.OrderId, (order, orderdetail) => new { order, orderdetail }).
                Where(x => x.order.SellerId == sellerId && stIds.Contains(x.orderdetail.OrderStateId) && x.order.RtableId != null)
                .GroupBy(x => new RtableState
                {
                    RtableId =

                x.order.RtableId ?? 0,
                    OrderStateId = x.orderdetail.OrderStateId
                })
                .Select(x => new RtableState { RtableId = x.Key.RtableId, OrderStateId = x.Key.OrderStateId }).ToListAsync();


            var rtableStates = rtableStatesServer.GroupBy(r => r.RtableId,
                        (key, value) => new RtableState
                        {
                            RtableId = key,
                            OrderStateId = value.OrderByDescending(x=>x.OrderStateId).Select(x => x.OrderStateId).FirstOrDefault()
                        }).ToList();

Answer 1

如异常消息所示，问题是由表达式

引起的

x.OrderByDescending(y => y.orderdetail.OrderStateId)
    .Select(y => y.orderdetail.OrderStateId)
    .FirstOrDefault()

其中 x 是由 GroupBy 运算符生成的 IGrouping<,>。

This may indicate either a bug or a limitation in EF Core.

我认为这是一个限制，可能永远不会修复，因为 GroupBy 包含除键和聚合表达式以外的表达式的结果没有自然的 SQL 等价物。

一般的解决方案是尽可能避免 GroupBy 并使用具有相关子查询的替代构造。但是这个特定的查询有简单的自然解决方案，因为表达式

set.OrderByDescending(item => item.Property).Select(item => itm.Property).FirstOfDefault()

可以用

表示

set.Max(item => item.Property)

这是一个标准（因此支持聚合）。

将上述有问题的表达式替换为

x.Max(y => y.orderdetail.OrderStateId)

问题就迎刃而解了

无法翻译 EF Core 3 Linq

EF Core 3 Linq could not be translated

c#

entity-framework-core

ef-core-3.0