entity-framework - 带投影的 EF4 Include()

Question

我正在尝试做一个简单的查询，它涉及预加载和投影，但遇到了问题。我正在使用 CodeFirst CTP5，但我相信这个问题也会影响直接 EF4。

这是我的初始查询:

public List<ArticleSummary> GetArticles()
    {
        var articlesQuery = _db.Articles.Include(article => article.Category).Select(article => new ArticleSummary
        {
            Article = article,
            CommentsCount = article.Comments.Count
        });

        return articlesQuery.ToList();
    }

这导致文章的类别属性为空。如果我取出投影，它就可以正常工作。看完this ，好像提示我需要在投影之后做include，所以我把query改成了:

    public List<ArticleSummary> GetArticles()
    {
        var articlesQuery = _db.Articles.Select(article => new ArticleSummary
        {
            Article = article,
            CommentsCount = article.Comments.Count
        });

        articlesQuery = articlesQuery.Include(x => x.Article.Category);

        return articlesQuery.ToList();
    }

这会导致类似于 this SO post 的异常(见下文) .

"Unable to cast the type 'System.Linq.IQueryable1' to type
'System.Data.Objects.ObjectQuery1'. LINQ to Entities only supports casting Entity Data Model primitive types."

那么，我该怎么做呢？

Answer 1

你可以试试:

public List<ArticleSummary> GetArticles()
{
    var articlesQuery = _db.Articles.Select(article => new
        {
            Article = article,
            Category = article.Category
            CommentsCount = article.Comments.Count
        }
    ).ToList().
    Select(
       x => new ArticleSummary() 
       {
            Article = x.Article,
            CommentsCount = x.CommentsCount
       }
    );

    return articlesQuery.ToList();
}