Question

I have an extension method defined like so:

public static TSource MaxBy<TSource, TResult>(this IEnumerable<TSource> collection, Func<TSource, TResult> func) where TSource : class
{
    var comparer = Comparer<TSource>.Default;
    TSource maxItem = null;
    foreach (var item in collection)
    {
        if (comparer.Compare(item, maxItem) > 0)
            maxItem = item;
    }
    return maxItem;
}

which I then use in the following LINQ-to-Entities query:

var balancesQuery = from t in db.Transactions
                    where t.UserId == userId
                    group t by t.CurrencyCode into tg
                    let tMaxDate = tg.MaxBy(i => i.TsCreate)
                    join c in db.Currencies on tg.Key equals c.CurrencyCode
                    select new { Currency = c, Balance = tMaxDate.Balance }

So what I’m doing is – get the newest transaction (MaxBy TsCreate) in each currency (group by CurrencyCode) and then select the balance against each of those transactions.

My problem is – this does not work with Entity Framework (LINQ-to-Entities; I get:

LINQ to Entities does not recognize the method ‘Transaction MaxBy[Transaction,DateTime](System.Collections.Generic.IEnumerable'[Transaction], System.Func’2[Transaction,System.DateTime])’ method, and this method cannot be translated into a store expression.

The same query works with LINQ to SQL.

My questions are:

1. Is there a way to make it work with Entity Framework?
2. Or maybe there is a better way of querying for the same information, which would work with Entity Framework?

Thanks in advance for any help!

Answer 1

Isn’t the query same as:

var balancesQuery = from t in db.Transactions
                    where t.UserId == userId
                    group t by t.CurrencyCode into tg
                    join c in db.Currencies on tg.Key equals c.CurrencyCode
                    select new { 
                        Currency = c, 
                        Balance = tg.OrderByDescending(i => i.TsCreate).Take(1).Balance 
                    };