LinQ OrderBy匿名值

Question

Following code is not working:

var x = from user in db.tblUsers
        select new
        {
            user.Id,
            user.FirstName,
            user.LastName,
            NumOfWins = SqlFactory.GetNumberOfWinsByUser(user.Id)
        };

TblBindingSource.DataSource = x.OrderByDescending(user => user.NumOfWins);
dataGridView1.DataSource = TblBindingSource.DataSource; //<-- fail here

Error:

Method 'Int32 GetNumberOfWinsByUser(Int32)' has no supported translation to SQL.

I know that SqlFactory.GetNumberOfWinsByUser(user.Id) returns a good value. Can anyone help me on this? Thanks in advance.

Answer 1

The answer is in the error. You are obviously using a linq providor such as entity framework that translates your linq into SQL. It can't translate the method SqlFactory.GetNumberOfWinsByUser(user.Id) into a SQL statement. If you want to use it you will have to do something like :

from user in db.tblUsers.ToList()
          select new { user.Id, user.FirstName, user.LastName, NumOfWins =     SqlFactory.GetNumberOfWinsByUser(user.Id) };

That will force it to fetch the data into memory first and then apply the transform in your select statement.

I suspect that the above is very inefficient but that is just a guess.

Answer 2

There is a way to do this kind of embedding of functions into Linq2SQL queries, provided your function ( GetNumberOfWinsByUser() in your case) can itself be written as Expression<Func<>> type where the expression can be converted to SQL by the provider

This was devised by Tomáš Petříček and described on his blog:

http://tomasp.net/blog/dynamic-linq-queries.aspx

It explains how you can defined an expression:

Expression<Func<Nwind.Product, decimal?>> calcPrice = 
  (p) => p.UnitPrice * 1.19m;

and then embed that into a Linq to SQL query:

var q = 
   from p in db.Products.ToExpandable()  
   where calcPrice.Expand(p) > 30.0m
   select new { 
     p.ProductName, 
     OriginalPrice = p.UnitPrice,
     ShopPrice = calcPrice.Expand(p) };

Using an extension method ToExpandable() that he created (the code is available for download).

This allows the entire query to be parsed into SQL and executed on the DB server without having to materialise any partial results sets into memory. It should be pretty efficient and can be made to handle very complex queries. I generally found that encapsulating the logic into expressions liek this led to simple code with good re-usability.

I used this extensively in a project and it works very well. You need to have a good grasp of expressions though.