Entity framework performance when using methods in the select clause - entity-framework

Question

I'm looking at the SQL generated when performing simple select queries. I'm using code first with the sample blog context from nuget.
If the following is run:
BlogContext _context = new BlogContext();
var comments = _context.Comments.Select(c => new CommentReadOnly {Author = c.Author});
var count = comments.Count();
The following SQL is produced:
SELECT
[GroupBy1].[A1] AS [C1]
FROM ( SELECT
COUNT(1) AS [A1]
FROM [dbo].[Comments] AS [Extent1]
) AS [GroupBy1]
Where the count is performed in the SQL which is expected.
However if I change the code to look like this:
BlogContext _context = new BlogContext();
var comments = _context.Comments.Select(c => new CommentReadOnly {Author = c.Author});
var count = comments.Count();
private CommentReadOnly ToCommentReadOnly(Comment comment)
{
return new CommentReadOnly
{
Author = comment.Author,
};
}
The following SQL is produced:
SELECT
[Extent1].[ID] AS [ID],
[Extent1].[PostID] AS [PostID],
[Extent1].[Text] AS [Text],
[Extent1].[Author] AS [Author]
FROM [dbo].[Comments] AS [Extent1]
With the count done in code.
The reason (I think) is because the first is returned as IQueryable where as the second is IEnumerable.
Is it possible to return the second query as IQueryable without executing the SQL?
The reason I ask is that I'm creating a generic repository layer that can query my entities and convert them to the required type (in the example above comment might have a couple of different 'readonly' objects). I don't want the SQL executing so early as paging may be done or other filtering in different situations.

Answer 1

I don't see any difference in those two queries. However, I guess you want to return a IQueryable object to the client so the client can perform further filtering and get the count from there.
You can simply return the object without doing the select and let the client do the rest.
return _context.Comments
The client can perform additional filtering on this IQueryable object

Answer 2

I think in your second query you execute the function ToCommentReadOnly() so this can't be done entirely in SQL and you end up with a Linq To Objects (IEnumerable).
But you state that you want to return an IQueryable from your Repository. This is not a recommended practice! The code to access the data should be hidden inside your repository otherwhise you will run into problems.
Say for example that your repository (which encapsulates your ObjectContext) goes out of scope after which you try to enumerate the IQueryable result the repository gave you. This will throw an error because the IQueryable can't be executed anymore.
If you expose an IQueryable from your Repository you give the end user of your Repository all the freedom they want in building their own queries, which is the thing you want to avoid by adding a repository!
So returning an IEnumerable from your Repository is a good thing :)