LINQ MVC ViewModel：使用可选字段多次连接到同一个表

Question

给定以下数据库结构

类别

身份证 类别名称ResID 父类别（可选）

资源

身份证 文本 朗

并给定一个 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; }
}

我想获取包含 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得 ParentCategoryName 而无需再次迭代，这绝对是错误的。

Answer 1

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();

Answer 2

假设您的表格中有以下数据

dbo.Categories

ID  CategoryNameResID   ParentCategory
1   1                   NULL
2   2                   NULL
3   3                   1
4   4                   NULL
5   5                   4
6   6                   4
7   7                   4


dbo.Resources

ID  Text                Lang
1   Standard            en-GB
2   Custom              en-GB
3   Standard Oversize   en-GB
4   Loose               en-GB
5   Loose 2F Set        en-GB
6   Loose (4” Scale)    en-GB
7   Loose (6” Scale)    en-GB

以下 LINQ 语句将输出所需的结果：

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; }
}   


var categories = (from cat in Categories
                  join res in Resources on cat.CategoryNameResID equals res.ID let categoryName = res.Text
                  select new CategoryViewModel
                  {
                    ID = cat.ID,
                    CategoryNameResID = cat.CategoryNameResID,
                    CategoryName = categoryName,
                    ParentCategory = cat.ParentCategory,    
                    ParentCategoryName = Resources.FirstOrDefault(r => r.ID == cat.ParentCategory).Text
                  }).ToList();


foreach(var c in categories)
{
   Console.WriteLine(c.CategoryName + " - " + c.ParentCategoryName);   
}

// Prints
Standard - 
Custom - 
Standard Oversize - Standard
Loose - 
Loose 2F Set - Loose
Loose (4” Scale) - Loose
Loose (6” Scale) - Loose