EF核心: LINQ选择“多对多-多对多”到“多对多”

Question

我有一个表格“参考”和一个表格“文章”，其中一篇文章引用其他文章。

我确实有一些简单的参考资料，比如:A -> B

SQL：

select ab.*
from Article a
inner join Reference ab on ab.ArticleFromId = a.Id
inner join Article b on b.Id = ab.ArticleToId
where a.ArticleNo = "1234"

C# LINQ：

_context.Reference
   .Where(r => r.ArticleFromNavigation.ArticleNo.Equals("1234"));

我也有引用链，比如:a -> B -> C(假设一条链中最多只有3篇文章)

SQL：

select ab.ArticleFromId, bc.ArticleToId
from Article a
inner join Reference ab on ab.ArticleFromId = a.Id
inner join Article b on b.Id = ab.ArticleToId
inner join Reference bc on bc.ArticleFromId = b.Id
inner join Article c on c.Id = bc.ArticleToId
where a.ArticleNo = "1234"

这在SQL中很容易，因为结果只是与附加联接相乘，但我不知道如何用LINQ编写它。

我希望它是这样的(这是行不通的)：

_context.Reference
   .Where(r => r.ArticleFromNavigation.ArticleNo.Equals("1234"))
   .Select(r => new Reference
   {
       ArticleFromNavigation = r.ArticleFromNavigation, //this is article "A"
       ArticleToNavigation = r.ArticleToNavigation.ReferenceArticleToNavigations //this wont work as it's a iCollection
   }).AsNoTrackable();

在这里，我想要"A -> C“的”引用“类型的新结果。我想我必须包括/select/join/select/select多(？)收集之前的“新参考”-section，但我毫无头绪。

有什么办法可以把它存档吗？

Answer 1

好吧，您可以完全像在SQL中那样做，但是使用导航属性而不是联接。

我将使用LINQ查询语法，因为它更好地显示了相似性，而且对于这种类型的查询，方法语法非常复杂，很难阅读：

from a in _context.Article
from ab in a.ReferenceArticleFromNavigations
let b = ab.ArticleToNavigation
from bc in b.ReferenceArticleFromNavigations
let c = bc.ArticleToNavigation
where a.ArticleNo = "1234"
select new Reference
{
    ArticleFromNavigation = a,
    ArticleToNavigation = c,
}

let语句并不是非常需要的(您可以直接使用引用导航属性)，我将它们包括进来只是为了使LINQ查询更接近于SQL查询。

实际上，在这种情况下，方法等效并不是那么糟糕--使用嵌套的SelectMany来平平几个级别，并使用SelectMany重载来投影(顶部、底部)对，允许这样做：

_context.Article
    .Where(a => a.ArticleNo = "1234")
    .SelectMany(a => a.ReferenceArticleFromNavigations
        .SelectMany(ab => ab.ArticleToNavigation.ReferenceArticleFromNavigations)
        // include as many `SelectMany` like the above as you wish until you hit the desired level of nesting
        .Select(bc => bc.ArticleToNavigation),
    (a, c) => new Reference
    {
        ArticleFromNavigation = a,
        ArticleToNavigation = c,
    });

Answer 2

为了使语法正确，我将数据库建模为类。见下面的代码：

using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication107
{
    class Program
    {
        static void Main(string[] args)
        {
            Context _context = new Context();
            string ArticleNo = "1234";

            var results = (from a in _context.article.Where(x => x.Id == ArticleNo)
                           join ab in _context.reference
                              .Where(x => (x.ArticleFromId == x.ArticleToId))
                              on a.Id equals ab.ArticleFromId
                           select new { a = a, ab = ab }
                          ).Select(r => new Reference()
                          {
                              ArticleFromNavigation = r.a,
                              ArticleToNavigation = r.a.ReferenceArticleToNavigations.ToList() 
                          }).ToList();
        }
    }

    public class Context
    {
        public List<Reference> reference { get; set; }
        public List<Article> article { get; set; }
    }
    public class Reference
    {
        public string ArticleFromId { get; set; }
        public string ArticleToId { get; set; }
        public Article ArticleFromNavigation { get; set; }
        public List<string> ArticleToNavigation { get; set; }
    }
    public class Article
    {
        public string Id { get; set; }
        public List<string> ReferenceArticleToNavigations { get; set; }
    }

}