How do I get the most recent entry from a table using LINQ?

Question

I have a table of prices, which has a column for Date Updated. Every time there is a price change, a new entry is made to the table.

In order to return the most up to date price, I am trying to order by date, and then use .Last() to get the newest entry:

 var upgrades = from d in vf.tl_phn_devices
                       where d.available == true
                       select new
                       {
                           DeviceName = d.tl_phn_manufacturer.manufacturer + " " + d.model,
                           Price = (from p in vf.tl_phn_prices 
                                    where p.deviceID == d.deviceID
                                    orderby p.dateUpdated ascending
                                    select p.price).Last(),
                           URL = d.url,
                           ImageURL = d.image

                       };

However, when I run the above I get an error saying that .Last() is unsupported. I have also tried .AsEnumberable().Last() but this didn't work either.

Elsewhere in the code I got round a similar issue by taking an extra step:

  var orders = (from o in vf.tl_phn_orders
                    where o.login == CurrentUserName
                    orderby o.date ascending
                    select new
                     {
                         Login = o.login,
                         Name = o.name,
                         Device = o.tl_phn_device.tl_phn_manufacturer.manufacturer + " " + o.tl_phn_device.model,
                         Price = o.tl_phn_price.price,
                         date = o.date

                     }).AsEnumerable();

        var order = orders.LastOrDefault();

But I want to do it all in one "hit" so I can return it in the first query.

Answer 1

Just do orderby descending and then Select First .

Last and LastOrDefault are not supported with Entity framework, since they can't get translated into underlying language (SQL for your case)

orderby p.dateUpdated descending
select p.price).First(),

You may also see: Supported and Unsupported LINQ Methods (LINQ to Entities)

Answer 2

Use First() and change the order:

 var upgrades = from d in vf.tl_phn_devices
                where d.available
                select new
                {
                    DeviceName = d.tl_phn_manufacturer.manufacturer + " " + d.model,
                    Price = (from p in vf.tl_phn_prices 
                             where p.deviceID == d.deviceID
                             orderby p.dateUpdated descending
                             select p.price).First(),
                    URL = d.url,
                    ImageURL = d.image    
                };

The problem is that EntityFramework cannot generate the SQL for your query.

Your second query's use of .Last() works because you have loaded the entities into memory (with .AsEnumerable() ) and are now using LINQ to Objects instead of LINQ to Entities.

You cannot do the same with the first query because of the nested queries.