How to get a list of the grouped values in linq groupby?

Question

Linq newbie here, struggling with my first GroupBy query.

I have a list of objects of type KeywordInstance which represents a keyword, and the ID of the database record to which the keyword was applied.

Keyword      RecordID
macrophages  1
macrophages  2
cell cycle   3 
map kinase   2
cell cycle   1

What I want is a collection of all keywords, with a list of the RecordIDs to which each keyword was applied.

Keyword     RecordIDs 
macrophages 1, 2
cell cycle  1, 3
map kinase  2

I tried using Linq to get it into a new object. I only managed to get the distinct keywords.

var keywords = allWords .GroupBy(w => w.keyword) .Select(g => new {keyword = g.Key});

The problem is that I can't seem to get the values of g in any way. g is of the type IGrouping<String, KeywordInstance> and by documentation, it only has the property Key, but not even the property Value. All the examples I have seen on the Internet for groupby just tell me to select g itself, but the result of

var keywords = allWords
    .GroupBy(w => w.keyword)
    .Select(g => new {keyword = g.Key, RecordIDs = g});

is not what I want.

screenshot of result

Any try to get something out of g fails with the error message System.Linq.IGropuing<string, UserQuery.KeywordInstance> does not have a definition for [whatever I tried].

What am I doing wrong here?

I can imagine that this question is a duplicate, but I must have searched for the wrong keywords, at least the search results I found did not help me. — Rumi P., Nov 06 '13 at 13:12
I dont see the problem-- you have a key, and all the records belonging to that key? What where you expecting the records of `g` to look like :) ? — Jens Kloster, Nov 06 '13 at 13:16
@JensKloster I don't care what the records of g look like. What I need is a data structure which maps the list of integers '{3, 169, 2179, 2188, 2334, 2389, 2496}' to the key 'FACTOR RECEPTOR'. I thought I would use groupby, and obviously used it wrong, getting g out of it instead of what I needed. So the question is, how do I use it right? — Rumi P., Nov 06 '13 at 13:22

score 39 · Accepted Answer · answered Nov 06 '13 at 13:27

39

I think you are close to you solution.

var keywords = allWords
    .GroupBy(w => w.keyword)
    .Select(g => new 
           { 
              keyword = g.Key, 
              RecordIDs = g.Select(c => c.ID)
           });

Just Select the records you need.
The reason you are seeing the Keyword-column as well as the ID-column, is becuase it's part of g

answered Nov 06 '13 at 13:27

Jens Kloster

11,099
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thank you, I was trying to do it with something like RecordIDs = g.Values.ID and never thought of using a nested query on g. – Rumi P. Nov 06 '13 at 13:35
This does not seem to work when querying with Entity Framework. I had to do a "where" to select records by each group key. – ed22 Dec 29 '21 at 12:14

score 1 · Answer 2 · answered Nov 21 '22 at 04:01

1

var keywords = allWords.GroupBy(w => w.keyword);

foreach (var itm in keywords)
{
   var list = itm.ToList();  
   //list returns all of the original properties/values objects from allwords.
   //itm.key returns w.keyword
}

answered Nov 21 '22 at 04:01

John Foll

121
1
10

score 0 · Answer 3 · edited Aug 28 '23 at 20:51

0

One way to get values (but you lose the Key) is like the following:

var keywords = allWords
    .GroupBy(w => w.keyword)
    .Select(g => g.FirstOrDefault());

Or a cleaner approach:

var keywords = allWords
    .Select(w => w.keyword)
    .Distinct();

edited Aug 28 '23 at 20:51

halfer

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answered Jul 14 '23 at 16:25

Hooman Limouee

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How to get a list of the grouped values in linq groupby?

3 Answers3

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