How to implement SkipWhile with Linq to Sql without first loading the whole list into memory?

问题

I need to order the articles stored in a database by descending publication date and then take the first 20 records after the article with Id == 100.

This is what I would like to do with Linq:

IQueryable<Article> articles = 
    db.Articles
    .OrderByDescending(a => a.PublicationDate)
    .SkipWhile(a => a.Id != 100)
    .Take(20);

However, this generates a NotSupportedException because SkipWhile is not supported in Linq to Sql (see here).

A possible solution is to execute the query and then apply SkipWhile using Linq to Object:

IEnumerable<ArticleDescriptor> articles = 
    db.Articles
    .OrderByDescending(a => a.PublicationDate)
    .ToList()
    .SkipWhile(a => a.Article.Id != 100)
    .Take(20);

But this means I need to load the whole ordered list into memory first and then take 20 articles after the one with Id == 100.

Is there a way to avoid this huge memory consumption?

More in general, what is the best way to achieve this in SQL?

回答1:

If, as I'm guessing from the column name, PublicationDate doesn't change, you can do this in two separate queries:

• Establish the PublicationDate of the Article with Id == 100
• Retrieve the 20 articles from that date onwards

Something like:

var thresholdDate = db.Articles.Single(a => a.Id == 100).PublicationDate;
var articles = 
    db.Articles
    .Where(a => a.PublicationDate <= thresholdDate)
    .OrderByDescending(a => a.PublicationDate)
    .Take(20);

It might even be that LINQ to SQL can translate this:

var articles = 
    db.Articles
    .Where(a => a.PublicationDate 
             <= db.Articles.Single(aa => aa.Id == 100).PublicationDate)
    .OrderByDescending(a => a.PublicationDate)
    .Take(20);

but that may be too complex for it. Try it and see.

回答2:

You can try like this

var articles = 
    db.Articles
    .Where(a => a.PublicationDate < db.Articles
                                    .Where(aa => aa.Id==100)
                                    .Select(aa => aa.PublicationDate)
                                    .SingleOrDefault())
    .OrderByDescending(a => a.PublicationDate)
    .Take(20);

回答3:

Isnt the solution to just add a where statement?

IQueryable<Article> articles = db.Articles.Where(a => a.id != 100).OrderByDescending(a => a.PublicationDate).Take(20);

来源：https://stackoverflow.com/questions/9227828/how-to-implement-skipwhile-with-linq-to-sql-without-first-loading-the-whole-list