LINQ查询，选择分组依据的最新值-不支持的方法

Question

I have this table with fund data values and I want to select the latest values for each fund (FundId). This query is giving me problems though when executed. "Specified method is not supported".

var q = from f in ctx.FundDatas
group f by f.FundId into g
let latestDataItem = g.OrderByDescending(r => r.DateOfValue).FirstOrDefault()
select new { 
    g.Key, LatestDataItem = latestDataItem 
};
var list = q.ToList(); //Executed and exception is thrown

Why wouldn't this order by work? I don't want to just get the Key and DateOfValue, if so I would have just skipped the "let" part and made the select like this:

select new { 
    g.Key,
    LatestDateOfValue = g.Max(y=>y.DateOfValue)
};

The above works... But I want the whole object of the latest of each fund data items, not just the max date.

Here's the inner exception stack trace:

[NotSupportedException: Specified method is not supported.]
MySql.Data.Entity.SqlGenerator.Visit(DbApplyExpression expression) +28
System.Data.Common.CommandTrees.DbApplyExpression.Accept(DbExpressionVisitor`1 visitor)     +25
MySql.Data.Entity.SqlGenerator.VisitInputExpression(DbExpression e, String name,      TypeUsage type) +35
MySql.Data.Entity.SelectGenerator.VisitInputExpressionEnsureSelect(DbExpression e, String name, TypeUsage type) +21
MySql.Data.Entity.SelectGenerator.Visit(DbProjectExpression expression) +38
System.Data.Common.CommandTrees.DbProjectExpression.Accept(DbExpressionVisitor`1 visitor) +25
MySql.Data.Entity.SelectGenerator.GenerateSQL(DbCommandTree tree) +60
MySql.Data.MySqlClient.MySqlProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree) +376
System.Data.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree) +125
System.Data.EntityClient.EntityCommandDefinition..ctor(DbProviderFactory storeProviderFactory, DbCommandTree commandTree) +442

I'm running MySQL with .NET connector 6.5.4.0.

Edit: Table definition:

FundId      int(6), PK
DateOfValue date, PK
Value       double(12,6)

Answer 1

There's a known bug in the x.5.4 versions of the connection on the FirstOrDefault() method.

Refer http://bugs.mysql.com/bug.php?id=67377

 While executing the following LINQ. It used to work in the version 6.3.5: 

 entities.Reclamation.Where(x=> x.idUser == idUser).Select(x=> new {
    id = x.id,
    ReclamationReport = x.Report.Count == 0 ? null : x.Report.FirstOrDefault().id
    });

I'm not sure if this is the cause of your issue, as the bug reporter doesn't appear to be using the same pattern as you (although my Linq is not up to scratch so I don't even know if I'm comparing apples with apples), and while the exception is similar in implied meaning it is not identical (although bug reported in v6.6.4, so that might explain the subtle difference to what you're seeing in v6.5.4).

Refer to this maintenance release article, for the release applicable to 6.5.4:

https://blogs.oracle.com/MySqlOnWindows/entry/mysql_connector_net_6_55

Fix for method FirstOrDefault not supported in some LINQ to Entities queries (MySql bug #67377, Oracle bug #15856964).

So, I would recommend applying the maintenance release. .