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Creating reusable Entity Framework queries thanks to deferred execution

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With all the shenanigans that are now going on in my queries (see Sub-selects in code with the Entity Framework, Projection blows includes in Entity Framework and Sorting associations in the Entity Framework) it became important to follow the DRY (Don't repeat yourself) principle. This is surprisingly easy thanks to deferred execution. I can create a method like so:

private static IQueryable<PostData> AdvancedPostQuery(this BlogEntities be)
{
return from p in be.Posts
select new PostData
{
Post = p,
Tags = p.Tags,
Comments = p.Comments,
User = p.User,
NextLinkData = (from n in be.Posts where n.PostedDate > p.PostedDate orderby n.PostedDate ascending select new PostLinkData { Title = n.Title, LinkTitle = n.LinkTitle }).Take(1).FirstOrDefault(),
PreviousLinkData = (from prev in be.Posts where prev.PostedDate < p.PostedDate orderby prev.PostedDate descending select new PostLinkData { Title = prev.Title, LinkTitle = prev.LinkTitle }).Take(1).FirstOrDefault(),
};
}

And I can still add additional where clauses and the like to the returned IQueryable object without fear of loading the whole database into memory and performing those operations in-proc. No sirree, the execution (and generation) of the SQL query is deferred until the data is actually required for use in-proc which allows you to create a reusable method like the one above and build up your queries in layers.

Very nice. Very clever.

I've painted what some might consider a very negative picture of EF in these recent posts but I certainly don't view EF negatively. In fact, it's one of my favourite technologies to come out of Redmond for some time and I'm much happier with it than the previous manual DAL that powered thejoyofcode.com. Give it a shot.

Originally posted by Josh Twist on 13 Feb 2009 here.