在ASP.net中使用组合键展平嵌套分组的Linq查询

Question

In my database there are player-objects which do have a nationality and points.

I need a possibility to nest my groupings. Because they are given by the client, grouping into an anonymous key seems no option here. So I have to nest them like

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(/*...*/);

At the end, the Dictionary should contain the keys as string like ["USA|true"] , ["USA|false"] or ["GER|true"] with their respective elements.

I guess SelectMany is the key but I don't get the point where to start from to achieve this.

Answer 1

What about this solution :

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

Answering your concrete question.

As you mentioned, SelectMany is the key. The place in your query is right after the Select :

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

It can also replace the Select (ie start right after the first GroupBy ):

.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);