在 EF Core 中选择分组项

Question

这在 EF 6.4 中有效：

from a in Addresses
group a by new {a.StreetName, a.StreetNumber} into agrp
where agrp.Count() > 3
from aitem in agrp
select aitem

如果 EF Core 5 我得到：

InvalidOperationException: The LINQ expression 'agrp => agrp' 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 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

为什么？有没有不同的写法？

Answer 1

如果您同意将数据加载到内存中，一个简单的解决方案可能是在 Addresses:

之后添加 .ToList() 或 .AsEnumerable()

from a in Addresses.ToList() // or .AsEnumerable()
group a by new {a.StreetName, a.StreetNumber} into agrp
where agrp.Count() > 3
from aitem in agrp
select aitem

请注意，这（在 SqlServer 中）转换为：

SELECT [a].[Id], [a].[StreetName], [a].[StreetNumber]
FROM [Addresses] AS [a]

在 EF Core 中，GroupBy（在许多情况下）未转换为 SQL，但在内存中为运行。（为避免不小心将大量数据加载到内存中，EF 将抛出异常，除非调用 .ToList() 或 .AsEnumerable() 以表明这是故意的。）

(...) Since no database structure can represent an IGrouping, GroupBy operators have no translation in most cases. When an aggregate operator is applied to each group, which returns a scalar, it can be translated to SQL GROUP BY in relational databases. (...)

- Complex query operators, GroupBy

该文章还有一个查询示例，该查询转换为 group by，并在 Count 上进行筛选（包括在下方）。不幸的是，该示例并未完全涵盖问题中的示例。它不会 return 相关的 Address-objects，只有 group-by Key 和 Count。

var query = from p in context.Set<Post>()
            group p by p.AuthorId into g
            where g.Count() > 0
            orderby g.Key
            select new
            {
                g.Key,
                Count = g.Count()
            };

SELECT [p].[AuthorId] AS [Key], COUNT(*) AS [Count]
FROM [Posts] AS [p]
GROUP BY [p].[AuthorId]
HAVING COUNT(*) > 0
ORDER BY [p].[AuthorId]

Answer 2

这似乎有效：

var addressGroupQuery = from a in Addresses
                        group a by new {a.StreetName, a.StreetType} into agrp
                        where agrp.Count() > 3
                        select agrp.Max(a => a.AddressID);
                
var addresses = from a in Addresses
                where addressGroupQuery.Contains(a.AddressID)
                select a;

请注意，addressGroupQuery 仍然是一个延迟查询（没有 ToList）。也生成一些干净的外观 SQL：

SELECT [a].[AddressID], [a].[City], [a].[Country], [a].[StreetName], [a].[StreetNumber], [a].[StreetType], [a].[UnitNumber]
FROM [Address] AS [a]
WHERE EXISTS (
    SELECT 1
    FROM [Address] AS [a0]
    GROUP BY [a0].[StreetName], [a0].[StreetType]
    HAVING (COUNT(*) > 3) AND (MAX([a0].[AddressID]) = [a].[AddressID]))

在 EF Core 中选择分组项

Selecting Grouped Items in EF Core

linq-to-entities

entity-framework

entity-framework-core