LINQ Group-by clause on Enumerable field

Question

I'm running into a bit of a problem writing a LINQ Query.

Given this data

Object ID Items ------ -- ----- Object1 1 [1,2,4] Object2 2 [2] Object3 3 [2,4] Object4 4 [1]

I want to group on the IDS inside my Items property

The end result should be something like this

ItemID Objects ------ ------------------------------------ 1 [Object1, Object2, Object3, Object4] 2 [Object1, Object2, Object3] 4 [Object1, Object3]

In which "Objects" contains the entire "object", or the properties that I wish to select.

Any help with this? It's probably pretty basic, but I can't wrap my head around it right now.

Can you explain the logic how to transform the first table to the second? — Hamlet Hakobyan
– Hamlet Hakobyan, Commented Sep 22, 2013 at 21:40
how do you know which ItemIDs to look for? is it always from 1 to (known) n? union of all Items lists? — olydis
– olydis, Commented Sep 22, 2013 at 21:41

Tilak · Accepted Answer · 2013-09-22 21:42:17Z

Try following pseudocode

var result = objects.SelectMany ( o=>o.Items.Select (x=>new {item=x,objectid=o.object)).GroupBy (x=>x.item).ToArray() // objects -> collection of objects in question //Items.Select => create list of item to object mapping //objects.SelectMany -> Flattens item to object mapping into IEnumerable //.GroupBy => groups the result by itemid

Thanks, this worked. Late accept because I was cursing at the code until I finally noticed I erroneously put a wrong bracket somewhere.

MarcinJuraszek · Accepted Answer · 2013-09-22 21:42:50Z

1

var results = from o in source from i in o.Items group o.Object by i into g select new { ItemId = g.Key, Objects = g.ToList() }

answered Sep 22, 2013 at 21:42

MarcinJuraszek

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1 Comment

Kippie Over a year ago

Glancing over the code, I assume it does the same as Tilak's code. I accepted his answer because I prefer LINQ method syntax over query syntax.

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