LINQ group by query help: Select another related table with key

Question

请帮我完成以下查询：

var results = from species in db.Species
              join cats in db.Categories on species.CategoryId equals cats.CategoryId
              join groups in db.Groups on cats.GroupId equals groups.GroupId
              group groups by groups into g
              select new GroupWithCategoryChildren
              {
                  Group = g.Key,
                  Categories = (cats from above query for this same g.Key group)???
              };

我可以使用 SQL 非常轻松地做到这一点，但我正在努力使用 LINQ。我想在 above/main 单个查询中得到每个 Group 以及加入它的 cats 的列表，理想情况下无需在单独的查询中再次重新查询该数据.

非常感谢

Answer 1

我不确定我是否理解正确，但我认为这就是您所需要的：代替 group groups by groups 执行 group cats by groups 然后 Key 将是 groups 组的值将是所有匹配的类别。

var results = from species in db.Species
              join cats in db.Categories on species.CategoryId equals cats.CategoryId
              join groups in db.Groups on cats.GroupId equals groups.GroupId
              group cats by groups into g
              select new GroupWithCategoryChildren
              {
                  Group = g.Key,
                  Categories = g.ToList()
              };

我用这个来测试它：

List<dynamic> Species = new List<dynamic>()
{
    new { A = "1", CategoryId = 1 },
    new { A = "2", CategoryId = 2 },
};

List<dynamic> Categories = new List<dynamic>
{
    new { B = "1", CategoryId = 1, GroupId = 1 },
    new { B = "2", CategoryId = 2, GroupId = 1 },
};

List<dynamic> Groups = new List<dynamic>
{
    new { C = "1", GroupId = 1 }
};

//result: { Group: { C = "1", GroupId = 1 }, Categories (1,2)}

LINQ group by query help: Select another related table with key

LINQ group by query help: Select another related table with key

c#

linq

linq-to-sql