Linq GroupBy and select other attributes as list

Question

How to group by Properties and select other properties as lists using LINQ?

I'm bad at explaining it so here's an example:

Lets say I have a list of C# objects that can be represented in JSON like this:

[
    { "id":123, "name":"John", "carId":1, "carAge":3 },
    { "id":123, "name":"John", "carId":2, "carAge":4 },
    { "id":4, "name":"Mary", "carId":3, "carAge":3 },
    { "id":4, "name":"Mary", "carId":4, "carAge":3 },
    { "id":123, "name":"John", "carId":5, "carAge":3 }
]

Among all those objects only carId is unique.

I would like to group them by person details and join those details with connected car details.

The end result would look like this but as c# objects:

[
    {"id":123, "name":"John",
        [
            {"carId":1, "carAge":3},
            {"carId":2, "carAge":4},
            {"carId":5, "carAge":3}
        ]
    },
    {"id":4, "name":"Mary",
        [
            {"carId":3, "carAge":3},
            {"carId":4, "carAge":3}
        ]
    }
]

Edit:

I'm stuck after

list.GroupBy(x => new { x.Id, x.Name })

because I don't need any aggregate functions but only selection.

I apologize for not posting more code, this is a simplified version, the real thing has more filtering and convoluted domain specific terminology.

Everything is done in memory - without DB to worry about.

Please show what you have tried. We can help from there. There are many sources to see how to group by and project — Gilad Green, Jul 20 '17 at 06:06
Providing a [mcve] would make things simpler, too. It looks like this is a simple `GroupBy` operation where you specific separate key and element selectors, but it would be easier to say for sure with code. — Jon Skeet, Jul 20 '17 at 06:08
https://stackoverflow.com/questions/2280183/how-to-group-by-custom-types-with-linq should help you. Seems like grouping by the composite key - from 2 values is what you need — Vladimir, Jul 20 '17 at 06:12
I understand and agree, but the actual code gets complex and I think would distract from the problem, so unfortunately this is the best I can do right now. I made this oversimplified example just so I can explain my question better. — Tadija Bagarić, Jul 20 '17 at 06:15

Gilad Green · Accepted Answer · 2017-07-20T06:18:40.917

for each section of the group _ by _ expression create an anonymous object with the fields you want.

With any of the following ways you will construct a nested collection for each of your keys - when using a GroupBy it is not just about aggregations but about operations on specific groups of data. One case of it is aggregation methods.

var result = from item in data
             group new { item.carId, item.carAge }  by new {item.id, item.name} into g
             select g;

Another way would be

var result = from item in data
             group  by new {item.id, item.name} into g
             select new {
                 g.Key,
                 Data = g.Select(i => new {i.carId, i.carAge})
             };

Using method syntax as you tried you can use the following overload:

var result = data.Group(k => new {k.id, k.name},
                        e => new {e.carId, e.carAge});

Or if not the just add a Select for projection:

var result = data.Group(k => new {k.id, k.name})
                 .Select(g => new {
                     g.Key,
                     Data = g.Select(i => new {i.carId, i.carAge})
                 });

Linq GroupBy and select other attributes as list

1 Answers1

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