c# – LINQ MVC ViewModel：使用可选字段连接到同一个表

给出以下数据库结构

类别

ID
CategoryNameResID
ParentCategory(可选)

资源

ID
文本
郎

并给出了一个viewmodel

public class Categoryviewmodel
{
    public int ID { get; set; }
    public int CategoryNameResID { get; set; }
    public string CategoryName { get; set; }

    public int ParentCategory { get; set; }
    public string ParentCategoryName { get; set; }
}

我想获得包含ParentCategoryName的所有类别的列表

到目前为止我所做的是：

var categories = (from cat in db.Categories
                  join res in db.Resources on cat.CategoryNameResID equal res.ID
                  select new Categoryviewmodel{
ID = cat.ID,CategoryNameResID = cat.CategoryNameResID,CategoryName = res.Text,ParentCategory = cat.ParentCategory,ParentCategoryName = (from p in db.Resources
where p.ID == cat.ParentCategory
select p.Text)
}).ToList();

我无法弄清楚如何获取ParentCategoryName而不必再次迭代,这绝对是错误的.

解决方法

试试这个：

(from cat in cats
join res in resources on cat.ResId equals res.Id let categoryName = res.Text
join cat1 in cats on cat.ParentId equals cat1.Id into parentJoin
from pj in parentJoin.DefaultIfEmpty() let parentCatResId =pj==null?0: pj.ResId
join res1 in resources on parentCatResId equals res1.Id into resJoin
from res2 in resJoin.DefaultIfEmpty() let parentName = (res2==null?string.Empty:res2.Text)
    select new CategoryVM
    {
        Id = cat.Id,ResId = cat.ResId,CatName = categoryName,ParentId = cat.ParentId,ParentName = parentName
    }).ToList();