在 ASP.net 中使用组合键展平嵌套分组的 Linq 查询

Question

在我的数据库中，有些玩家对象确实具有国籍和分数。

我需要一个嵌套我的分组的可能性。因为它们是由客户端提供的，所以在这里分组为匿名密钥似乎没有选择。 所以我必须像嵌套它们一样

Players.Where(p => p.Created <= /*some date*/)
    .GroupBy(p => p.Nationality) // group them by their nationality
    .Select(arg => new {
        arg.Key,
        Elements = arg.GroupBy(p => p.Points > 0) // group them by the ones with points and the ones without
    })
    . // here i need to flatten them by also combining the key(s) of the groupings to put them into a dictionary
    .ToDictionary(/*...*/);

最后，Dictionary 应包含 string 等键，如 ["USA|true"]、["USA|false"] 或 ["GER|true"] 及其各自的元素。

我猜SelectMany 是关键，但我不知道从哪里开始实现这一点。

Answer 1

这个solution怎么样：

public class Player
{
    public string Nationality {get;set;}
    public int Points {get;set;}
    public double otherProp {get;set;}

    //new field is added
    public string groupings {get;set;}
}

var groups = new List<Func<Player, string>>();
groups.Add(x => x.Nationality);
groups.Add(x => (x.Points > 0).ToString().ToLower());
Players.ForEach(x =>
    groups.ForEach(y => x.groupings = x.groupings + (x.groupings == null ? "" : "|") + y(x))
);
var answer = Players.GroupBy(x => x.groupings).ToDictionary(x => x.Key, x => x.ToList());

Answer 2

回答您的具体问题。

正如您所提到的，SelectMany 是关键。您查询中的位置就在Select 之后：

.Select(...)
.SelectMany(g1 => g1.Elements.Select(g2 => new {
    Key = g1.Key + "|" + g2.Key, Elements = g2.ToList() }))
.ToDictionary(g => g.Key, g => g.Elements);

它也可以替换Select（即在第一个GroupBy之后开始）：

.GroupBy(p => p.Nationality)
.SelectMany(g1 => g1.GroupBy(p => p.Points > 0).Select(g2 => new {
    Key = g1.Key + "|" + g2.Key, Elements = g2.ToList() }))
.ToDictionary(g => g.Key, g => g.Elements);